THE  FRUIT  OF  OPUNTIA  FULGIDA 

A  STUDY  OF 
PERENNATION  AND  PROLIFERATION  IN 
THE   FRUITS   OF    CERTAIN   CACTACE/E 


BY 
DUNCAN  S.  JOHNSON 


QK4-95 

Cll 

J6 


PUBUSHED  BY  THE  CaRNEGIE  INSTITUTION  OF  WASHINGTON 
WASHINGTON.  1918 


®Ijp  i.  m.  'Ml  ICtbrarg 


Nnrtly  (Earoltna  ^tnU  Imnerattg 

Gil 
J6 


ATE    UNIVERSITY  I  IBRARIES 


lllli,       II 

S00555181    P 


Worth  Carolina 


ibrary 


THE  FRUIT  OF  OPUNTIA  FULGIDA 

A  STUDY  OF 
PERENNATION  AND  PROLIFERATION  IN 
THE   FRUITS    OF    CERTAIN    CACTACE/E 


BY 
DUNCAN  S:  JOHNSON 


PUBUSHED  BY  THE  CaRNEGIE  InSTITLT 
WASHINGTON.  V. 


THIS  BOOK  IS  DUE  ON  THE  DATE 
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CAENEGIE  INSTITUTION  OF  WASHINGTON 
Publication  Xo.  2G9 


BALTIMORE,    MD.,    U.    8.    A, 


CONTENTS. 

PAGE 

Comparison  of  the  fruit  of  Opuntia  fulgida  with  those  of  other  angiosperms. .  5 

Vegetative  structure  of  Opuntia  fulgida 8 

Reproductive  organs  of  Opuntia  fulgida 9 

Origin  and  structure  of  the  flovi^er 9 

Development  of  the  wall  of  the  ovary  with  its  tubercles 9 

Structure  and  fate  of  the  leaves  of  the  ovarian  wall 10 

Areoles  or  axillary  buds  of  the  ovarian  wall 11 

Number,  size  and  distribution  of  the  areoles 12 

Origin  of  the  growing  point  of  the  areole  and  sequence  of  initia- 
tion of  its  organs 12 

The  trichomes  of  the  areole 13 

The  nectaries  of  the  areole,  their  distribution  and  morphology. ...  14 
Spines  or  thorns  of  the  areole,  their  distribution,  structure,  and 

fate   15 

Bristles  or  spicules  of  the  areole,  their  number,  structure,  and 

fate    16 

The  perianth,  stamens,  style,   and   stigmas,  their  development  and 

abscission 17 

Morphology  of  the  ovary 20 

The  fruit:    Its  structure,  persistence,  and  fate,  normal  and  abnormal. ...  25 

The  fruit  at  the  time  of  abscission  of  the  perianth 25 

The  mature  fruit 27 

The  perennating  fruit:    Its  structure  and  secondary  growth 29 

The  seed :    Its  structure,  persistence,  and  germination 32 

Proliferation  of  flower  and  fruit 35 

Proliferation  from  attached  flower  buds 35 

Proliferation  of  persistent  attached  fruits 37 

Proliferation  of  fallen  fruits 38 

Causes  and  significance  of  perennation  and  of  the  diverse  types  of 

proliferation  41 

Proliferation  of  flower  or  fruit  in  allied  species 47 

Proliferation  of  joints  and  fruits  in  relation  to  the  sterility  of  fruits 51 

Summary  and  conclusions 53 

Literature  cited ..._• 56 

Explanation  of  plates % 57 


FRONTISPIECE 


A  mature  plant  of  Opuntia  fukjuhi  on  reservntion  of  Desert  Laboratory  at  Tucson,  showing 
a  frequent  type  of  forked  trunk,  due  to  injury  of  main  axis,  also  the  branching  habit 
and  clusters  of  fruit.  The  nesting  bird  is  the  cactus  wren,  Heleodytes  hrunneicapillm 
couesi  (Sharpe). 


THE  FRUIT  OF  OPUNTIA  FULGIDA. 

A  STUDY  OF   PERENNATION  AND  PROLIFERATION 
IN  THE  FRUITS  OF  CERTAIN  CACTACE/E.' 


By  DUNCAN  S.  JOHNSON. 


This  paper  embodies  a  discussion  of  the  occurrence  and  significance  of  a 
number  of  striking  peculiarities  in  the  development  and  fate  of  the  per- 
sistent, self-propagating  fruits  of  certain  opuntias.  The  discussion  will  be 
concerned  primarily  with  the  perennation  and  vegetative  propagation  of  the 
ovary  of  Opuntia  fulgida.  This  cactus  has  been  chosen  for  special  consid- 
eration because  of  its  most  remarkable  power  of  budding  off  secondary 
flowers  from  the  primary  ones  and  also  of  fonning  new  flowers  and  vegeta- 
tive shoots  from  the  long-persistent  fruits. 

This  investigation  has  been  aided  by  grants  from  the  Department  of 
Botanical  Research  of  the  Carnegie  Institution  of  Washington.  Acknowl- 
edgment is  here  made  to  Director  D.  T.  MacDougal  for  making  available 
to  the  writer  the  facilities  of  the  Desert  Laboratory  at  Tucson,  Arizona,  and 
of  the  Coastal  Laboratory  at  Carmel,  California.  The  principal  parts  of 
the  work  have  been  done  at  these  two  laboratories.  Other  portions  ol  it 
and  much  of  the  writing  of  this  paper  have  been  done  at  the  Harps-vN-ell 
Laboratory,  at  South  Harpswell,  Maine,  and  at  Johns  Hopkins  University. 
Acknowledgment  is  also  made,  for  aid  in  securing  material,  information,  or 
photographs  for  this  study,  to  Drs.  Forrest  Shreve  and  Hermann  Spoehr,  of 
tlie  Department  of  Botanical  Research  of  the  Carnegie  Institution,  to  Dr. 
David  Griffiths,  of  the  Bureau  of  Plant  Industry,  United  States  Department 
of  Agi-iculture,  to  Dr.  J.  'N.  Rose,  of  the  Smithsonian  Institution,  and  to  Dr. 
N.  L.  Britton,  of  the  INTew  York  Botanical  Garden. 

COMPARISON  OF  THE  FRUIT  OF  OPUNTIA  FULGIDA 
WITH  THOSE  OF  OTHER  ANGIOSPERMS. 

One  of  the  almost  universal  characteristics  of  the  fruit  in  augiosperms  is 
the  comparative  brevity  of  its  development.  The  whole  duration  of  this, 
from  the  inception  of  the  flower  to  the  ripening  of  the  fniit  and  it^  fiual 
separation  from  the  pareut  plant,  or  its  opening  for  the  discharge  of  its 
seeds,  is  usually  less  than  a  year,  often  very  much  less. 

The  phases  of  this  developmental  cycle  of  the  fruit  which  can  usually  be 
distinguished  are:  First,  a  j^eriod  of  initiation  and  maturing  of  the  parts 
of  the  flower,  which  opens  for  pollination  at  about  the  time  an  egg  has  been 

'  Botanical  contribution  from  The  Johns  Hoplvins  University,  No.  56. 

5 


6  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

formed  in  the  embryo  sac;  secondly,  there  follows  soon  after  pollination, 
which  is  usually  succeeded  by  fertilization,  a  period  of  active  vegetative 
growth  of  the  ovary  and  often  of  other  parts  which  are  to  enter  into  the 
make-up  of  the  fruit.  While  the  fruit  is  thus  growing  to  its  mature  size 
the  seeds  are  also  taking  on  their  characteristic  form  and  size.  Thirdly, 
accompanying  or  immediately  succeeding  the  final  maturing  of  the  seeds,  a 
process  of  ripening  occurs  in  the  fruit.  During  this  ripening  process  the 
outer  tissues,  the  niesocarp  and  epicarp  of  the  fruit,  may  soften  to  form  a 
juicy  pulp.  In  this  case  the  starches,  distasteful  glueosides,  acids,  tannins, 
alkaloids,  etc.,  which  are  often  present  in  the  cells  of  this  pulp  in  the  green 
fruit,  are  transformed  into  the  sugars,  mild  acids,  and  other  tasteful  flavors, 
and  often  also  into  brightly  colored  glueosides  that  make  the  fruits  attractive 
to  animals.  While  these  changes  are  taking  place  in  the  outer  layers  of  the 
fruit  the  inner  layer,  the  endocarp,  may  harden  to  form  the  firm  stone  that 
protects  the  seed  when  it  is  eaten  by  animals.  In  other  types  of  fruits,  as  in 
pods  and  capsules,  the  ripening  process  involves  a  drying  out  and  hardening 
of  all  the  tissues  of  the  fruit. 

Ripening  of  the  fruit  is  usually  followed,  often  very  promptly,  by  ita 
separation  from  the  plant  or  by  its  dehiscence  and  the  discharge  of  its  seeds. 
Either  of  these  two  fates  of  the  fruit  involves  the  more  or  less  immediate 
death  of  its  tissues,  aside  from  the  seeds.  In  a  few  fruits,  such  as  that  of 
palms  like  the  coconut  or  the  pomes  of  the  Eosace^e,  certain  tissues  of  the 
fruit  may  remain  alive  for  some  weeks  or  months  after  separation  from  the 
parent  plant.  The  growth  occurring  in  these  cases  is,  however,  compara- 
tively slight  and  it  does  not  give  rise  to  new  buds  or  new  plants. 

In  the  cases  of  certain  opuntias,  chiefly  cylindropuntias,  the  fruits  differ 
from  the  usual  type  characterized  above  in  several  very  remarkable  particu- 
lars. In  the  first  place  the  fruits  do  not  ripen  with  the  maturing  of  the 
seeds,  but  continue  to  grow  actively  without  undergoing  the  usual  softening 
and  change  of  color  and  of  chemical  composition  so  characteristic  of  the 
ripening  process  of  most  fleshy  fruits.  Secondly,  the  fruits  of  0.  fulgida 
are  not  shed  from  the  plant  when  the  seeds  are  ripe,  but  usually  remain 
firmly  attached  and  growing,  year  after  year.  Thirdly,  these  attached 
fruits  (or  even  the  unopened  flowers),  may,  in  situ,  give  rise  from  their 
axillary  buds  to  from  one  to  ten  secondary  flowers  and  fruits.  A  few  weeks 
later  these  secondary  flowers  may  give  rise  in  the  same  way  to  tertiary  ones 
and  these  in  turn  to  quaternary  ones.  Thus,  three  or  four  generations  of 
flowers  and  fruits  may  be  produced,  all  in  a  single  blooming  season  of  three 
or  four  months.  Tourthly,  if  the  fruits  become  separated  from  the  plant 
and  fall  on  moist  soil^  the  same  axillary  buds  which  in  the  attached  fruits 
would  form  nothing  but  flowers  will,  in  the  fallen  fruits,  give  rise  to 
vegetative  shoots  and  roots  and  to  these  only.  Finally,  the  embryos  of  the 
ripe  seeds,  inclosed  in  the  persistent,  attached  fruits,  retain  their  power  of 
germination  for  many  years. 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  7 

Certain  of  the  peculiarities  above  noted  in  the  frnits  and  seeds  of  Opuntia 
are,  it  is  true,  found  in  a  few  other  plants,  though  in  none  of  which  the 
writer  has  found  record  is  there  such  an  aggregate  of  unusual  features  of 
development,  and  certain  of  these  features  are  unlaiown  outside  the  genus 
Opuntia. 

The  persistent  attaclnnent  and  continued  gTo^^i;h  of  the  fruit  just  noted  is, 
as  far  as  I  can  learn,  recorded  for  but  one  other  family  of  angiosperms,  the 
Myrtacea).  In  the  genera  Callidemon  and  Melaleuca,  for  example,  the 
fniits  may  persist  for  10  or  15  years.  In  the  former,  according  to  Ewart 
(1907),  the  fruit  opens  and  discharges  the  living  seeds  only  when  it  has  been 
killed  by  the  cutting  off  of  its  water-supply.  This  latter  may  happen  in 
consequence  of  severe  drought,  of  the  breaking  off  of  the  branch  bearing  it, 
or  from  the  death  of  the  branch  or  whole  tree  from  fire  or  other  cause. 

There  are,  however,  three  important  differences  between  the  behavior  of 
the  fruits  of  this  Australian  "  bottle-brush  "  tree  and  those  of  such  opuntias 
as  the  "  choUa  "  (0.  fulgida).  In  the  first  place,  the  persistent  fruit  of 
Callistemon  does  not  possess  axillary  buds  and  therefore  does  not,  like 
Opuntia,  give  rise  to  secondary  and  tertiary  flowers  and  fruits  from  these. 
Secondly,  the  fruits  of  the  bottle-brush  tree,  though  they  may  open  and  dis- 
charge their  seeds  with  the  first  cutting  off  of  the  water-supply,  do  not  them- 
selves fall  from  the  tree  until  some  time  after  they  are  dead ;  hence,  they  can 
play  no  part  in  the  vegetative  propagation  of  the  species.  Thirdly,  the 
seeds  of  Callistemon  are  ultimately  shed  from  the  fruit  to  play  the  most 
important  role  in  the  dissemination  of  the  plant,  while  the  seeds  of  the  fleshy 
fruits  of  Opuntia  fulgida  are,  as  we  have  seen,  never  discharged  and  appar- 
ently rarely  germinate  under  natural  conditions. 

From  this  comparison  of  the  opuntias  with  the  only  other  family  of  plants 
having  fruits  with  similar  peculiarities,  it  is  clear  that  these  Cactacese  have 
become  much  more  abnormal  as  regards  the  behavior  of  propagative  struc- 
tures than  any  other  family  of  angiosperms.  Moreover,  the  readiness  with 
which  a  growing-point  destined  to  give  rise  to  a  flower  may  be  induced  to 
form  a  vegetative  shoot  (by  the  mere  separation  of  the  fruit  bearing  it  from 
the  parent  plant)  suggests  the  possibility  of  discovering  here  some  of  the 
causes  detennining  the  production  from  the  same  meristematic  mass,  in  one 
case  of  a  reproductive  organ  or  in  another  of  a  vegetative  shoot. 

With  this  much  of  suggestion  of  the  structures  and  phenomena  we  are  to 
deal  with,  we  shall  now  examine  more  closely  into  the  development  and  fate 
of  the  flower,  fruit,  and  seed  of  certain  opuntias.  We  shall  be  concerned 
primarily  with  those  of  Opuntia  fulgida,  in  which  these  structures  have  been 
studied  most  carefully  in  Tucson,  Carmel,  and  Baltimore.  Incidentally  we 
shall  also  note  the  structure  and  capacity  for  propagation  of  the  vegetative 
joints  of  certain  species,  in  order  that  we  may  compare  with  them  the 
structures  and  phenomena  observed  in  sprouting  fruits. 


8  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

VEGETATIVE  STRUCTURE  OF  OPUNTIA  FULGIDA. 

Opuntia  fulgida  is  a  tree-like,  Sonoran  species  of  Cylindropmvtia,  which 
commonlj  grows  to  2  or  3  meters  in  height  and  forms  a  rather  irregular  flat- 
topped  crown.  The  older  or  main  branches  are  horizontal,  or  ascendant  at 
their  bases,  but  bent  do^vn  at  their  tips  by  the  weight  of  the  thick  terminal 
branchlets  and  often  of  the  large  clusters  of  fruits  (fig.  1).  The  spiny 
trunk  is  dark  brown  in  color,  woody  in  texture,  and  may  reach  20  or  25  cm. 
in  diameter.  The  main  branches,  which  are  woody  like  the  trunk,  may 
become  8  or  10  cm.  in  diameter.  The  ultimate  branches  at  the  end  of  the 
first  season's  growth  are  often  3  to  5  cm.  in  diameter  and  15  to  25  cm. 
long.  These  younger  branches  taper  abruptly  at  the  ends  and  the  lateral 
surfaces  are  provided  with  prominent  and  somewhat  elongated  mammillae 
or  tubercles  (figs.  4,  5,  9a).  Each  tubercle,  at  this  time,  bears  at  its  upper 
end  from  7  to  12  sheathed  spines  and  the  growing-point  of  a  lateral  bud 
(figs.  4,  95).  The  number  of  spines  in  each  areole  steadily  increases  with 
age,  and  hence  a  branch  10  years  old  may  bear  50  spines  in  each  areole. 
The  leaf  to  which  the  areole  is  axillary  is  a  small  and  veiy  transient  structure 
which  (on  falling)  leaves  only  a  minute  scar,  like  that  to  be  seen  on  the 
fruit  (figs.  5,  47).  The  vascular  system  of  the  stem  is  net-like  in  arrange- 
ment, being  of  the  same  general  type  as  that  described  and  figured  by 
Ganong  (1894,  fig.  7).  During  the  first  year  the  bulk  of  the  new  joint  is 
made  up  of  the  mucilaginous  pulp  of  the  pith  and  cortex.  The  latter  has 
a  well-developed  photosynthetic  and  aerating  system  of  the  type  to  be 
described  in  dealing  with  the  fruit,  and  large  numbers  of  slime-cells  {cf. 
Wetterwald,  1889,  fig.  19).  Chloroplasts  are  abundant  throughout  the 
whole  thickness  of  the  cortex  of  the  stem  and  may  even  occur  within  the 
zone  of  woody  bundles,  as  happens  in  the  projecting  tubercles.  With  the 
increase  in  thickness  of  the  branch  the  woody  cylinder  seems  to  gTOW  in 
diameter  more  rapidly  than  the  fleshy  cortex.  The  latter  finally  becomes 
stretched  and  smoothed  out  and  the  cortex  of  the  mature  stem  is  compara- 
tively thin  and  dry  (fig.  2,  at  base). 

On  some  plants  of  this  species,  as  has  been  noted  by  Toumey  (1895),  cer- 
tain of  the  new  joints  may  remain  relatively  short  and  have  less  prominent 
tubercles  and  fewer  spines,  becoming  thus  rather  fruit-like  in  form  (fig.  7a). 
These  joints  are  readily  detached  and  on  moist  soil  may  give  rise  to  new 
plants  by  proliferation,  just  as  the  ordinary  vegetative  branches  of  this  and 
many  other  opuntias  may  do. 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  9 

REPRODUCTIVE  ORGANS  OF  OPUNTIA  FULGIDA. 

The  reproductive  structures  (flower,  fruit,  and  seed)  to  which  most  atten- 
tion has  been  paid  in  this  study,  will  be  described  in  some  detail  under  three 
captions:  (1)  the  origin  and  structure  of  the  flower;  (2)  the  fruit,  its 
structure,  jxirsistence,  and  fate,  normal  and  almorraal ;  (3)  the  seed,  its 
structure,  persistence,  and  germination. 

The  most  aberrant  features  of  the  development  of  these  reproductive 
organs  are :  the  structure  of  the  wall  of  the  submerged  ovary,  with  its  numer- 
ous axillary  buds ;  the  capacity  of  these  buds  to  initiate  secondary  flowers, 
either  immediately  before  the  primary  ones  are  open  or  later,  in  the  same  or 
succeeding  seasons ;  and  finally  the  ability  of  these  same  axillary  buds,  when 
the  fruit  is  detached,  to  form  adventitious  roots  and  shoots  and  thus  to 
initiate  new  plants. 

ORIGIN  AND  STRUCTURE  OF  THE  FLOWER. 

The  primary  flower  of  the  season  in  Opuntia  fulgida  arises  from  one  of 
the  upper  or  more  terminal  axillary  buds  or  areoles  of  a  last  year's  vegetative 
joint  or  from  an  areole  of  a  fruit  of  the  first,  second,  or  third  year  preceding. 
The  number  of  flowers  developed  on  any  one  joint  or  fniit  in  a  single  season 
ranges  from  1  to  5  or  more  (figs.  4,  9a).  In  the  case  of  the  fruits,  flowers 
may  be  formed  from  other  areoles  in  succeeding  years  until  as  many  as  10 
or  12  flowers  and  fruits  are  often  found  attached  to  a  single  pei*sistent  fruit 
(fig.  48).  The  primary  flowers  of  a  season  are  first  evident,  in  0.  fulgida 
growing  near  Tucson,  during  the  latter  half  of  April.  Open  flowers  are 
rarely  seen  before  the  middle  of  May.  The  first  flowers  to  appear  and  to 
open  are  those  developed  on  vegetative  branches.  In  early  May  1912,  the 
larger  flower-buds  on  vegetative  branches  of  one  plant  observed  were  21  mm. 
long,  while  the  longest  ones  on  a  persistent  fruit  of  the  same  plant  were  but 
7  mm.  New  flowers  continue  to  open  successively  all  through  the  summer 
up  to  the  middle  of  September.      (See  Tourney,  1898  ;  Lloyd,  1907 ). 

DEVELOPMENT  OF  THE  WALL  OF  THE  OVARY  WITH  ITS  TUBERCLES. 

The  very  young  flower-bud,  when  firet  pushing  out  of  the  tuft  of  trichomes 
and  spicules  of  the  areole,  is  a  rather  hemispherical  body  about  0.3  mm. 
in  diameter.  Its  dome-like  or  somewhat  conical  upper  end  is  fonned  by  the 
few  earlier  of  the  15  to  30  or  sometimes  40  leaves  that  are  finally  developed 
from  the  Avail  of  each  ovary  (figs.  4,  9a,  47).  These  leaves,  w^hen  first 
fonued,  arch  over  the  dome-shaped  growing-point  (figs.  13,  14).  As  the 
flower  grows,  the  earlier  leaves  are  pushed  outward  by  the  younger  ones 
arising  between  them  (figs.  17,  22).  The  axis  of  the  flower  becomes  elon- 
gated to  1^/4  times  its  diameter  and  its  surface  becomes  very  irregrular. 
Below  each  leaf  and  its  associated  bud  the  surface  of  the  wall  of  the  ovary 
protrudes  to  form  a  prominent  tubercle  or  mammilla  (figs.  14, 16,  20).  This 
tubercle  is  at  first  finger-like ;  later  it  projects  farthest  at  its  upjier  end  and 


10  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

narrows  to  nothing  at  its  lower  end,  giving  it  thus  a  rather  triangular  outline 
in  a  radial  section  of  the  fruit  (figs.  4,  17,  28). 

At  the  time  of  the  opening  of  the  flower  these  tubercles  are  from  6  to  10 
mm.  in  length,  are  4  or  5  mm.  wide,  and  project  2  or  3  mm.  at  the  top.  In 
the  younger  flower-bud  the  radial  width  of  the  tubercle  is  greater  in  propor- 
tion to  its  length,  longitudinal  to  the  ovary,  than  is  indicated  above,  while  in 
the  mature  fruit  the  projection  is  much  less.  In  morphological  nature,  this 
tubercle  of  the  opuntias,  as  has  been  sho\vn  by  Goebel  (1889,  p.  79),  is  the 
combined  product  of  the  growing  upward  together  of  the  leaf-base  and  the 
axillary  bud  above  it.  This  is  clearly  indicated  by  a  comparison  of  different 
stages  in  its  development  (figs.  12,  14,  15,  17).  The  upper  end  of  the 
tubercle  is  somewhat  circular  in  cross-section  (figs.  30,  49),  and  somewhat 
raised  at  the  margin  (figs.  21,  22  ;  cf.  also,  Wetterwald,  1889,  fig.  19).  At 
the  highest  point  of  the  abaxial  side  of  the  margin  is  borne  the  leaf,  while  the 
most  depressed  central  portion  of  the  end  of  the  tubercle  is  occupied  by  the 
flattish  growing-point  of  the  axillary  bud,  which  is  surrounded  by  the  rudi- 
ments of  trichomes,  spicules,  and  nectaries  developed  from  it  (figs,  dl),  12, 
15,  32,  50).  It  is  difficult  to  see  how  all  the  structures  above  a,  at  the  left  of 
the  growing-point  in  figure  50,  can  be  regarded  as  parts  of  a  single  leaf,  as 
they  apparently  would  have  to  be  if  the  view  of  the  morphology  of  the 
tubercle  held  by  Darbishire  (1904,  p.  395)  were  accepted. 

The  growing-point  of  the  flower,  like  that  of  the  vegetative  shoot,  may  be 
slightly  convex  in  form  during  the  period  of  the  initiation  of  the  wall  of  the 
ovary  with  its  tubercles  and  the  leaves  borne  by  them.  It  may  even  retain 
some  of  this  convexity  during  the  initiation  of  the  16  or  more  sepals  and 
petals  (figs.  12,  13,  14),  With  the  beginning  of  formation  of  the  stamens 
and  carpels,  however,  the  same  growing-point  becomes  depressed  to  form  a 
cup  narro^^ing  in  at  the  upper  margin,  about  which  the  numerous  (250) 
stamens  are  initiated.  Later  still,  the  margin  closes  in  to  form  the  carpels, 
which  unite  above  to  f  oi-m  the  roof  of  the  ovary,  and  finally  stretch  upward  to 
form  the  style  and  its  6  stigmas  (figs.  15,  16,  17). 

STRUCTURE  AND  FATE  OF  THE  LEAVES  OF  THE  OVARIAN  WALL. 

The  leaves  of  the  wall  of  the  ovary  are,  as  noted,  about  15  to  40  in  number. 
Each  leaf  is  approximately  conical  in  form,  has  a  slightly  flattened  base,  and 
is  curved  inward  above  to  end  in  a  sharply  pointed  tip  (figs.  4,  5,  11,  19,  23, 
47).  The  mature  leaf  of  the  ovary  is  only  3  or  4  imn.  long  and  but  1  or  1.5 
mm.  in  diameter.  It  is  abruptly  constricted  at  the  base  to  a  stalk,  which  is 
nearer  the  ventral  side  and  is  barely  a  third  the  diameter  of  the  part  of  the 
leaf  just  above  (figs,  19,  50).  The  leaves  of  the  lower  third  of  the  ovary 
do  not  attain  more  than  half  the  size  mentioned,  while  those  of  the  upper 
quarter,  which  are  more  or  less  appressed  against  the  sepals,  may  be  some- 
what longer  and  are  commonly  very  much  broadened  (figs.  23,  47,  at  right). 
Before  the  ovary  has  reached  half  the  size  it  attains  at  the  opening  of  the 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  11 

flower,  the  lowermost  of  tlie  15  to  40  leaves  of  the  ovary  have  dropped  off, 
many  of  them  withering  while  mere  rudiments,  half-grown  or  less  (fig.  20, 
lower  areole).  The  separation  occurs  at  the  constriction  mentioned,  and  by 
the  time  the  flower  is  well  opened  all  its  leaves  outside  the  calyx  have  fallen. 
This  separation  is  apparently  not  determined  by  a  definite  abscission  layer. 
The  dropping  of  the  leaf  is  followed  by  the  shrinking  together  of  the  short 
stump  of  the  leaf-stalk  and  later  by  the  formation  of  a  protecting  scar-tissue 
of  15  to  20  layers  of  corky  cells  (fig.  14).  Evidently  these  leaves  of  the 
wall  of  the  ovary,  being  relatively  few,  small,  and  transient,  are  able,  like  the 
similar  leaves  of  the  vegetative  joint,  to  play  only  a  very  subordinate  part  in 
the  photosynthetic  work  of  the  plant.  They  are  certainly  much  less  impor- 
tant in  this  work  than  the  abundant  and  permanent  photosynthetic  tissue  of 
the  wall  itself. 

In  correspondence  with  this  relatively  unimportant  photosynthetic  work 
of  the  leaf,  its  internal  structure  shows  little  of  the  characteristic  specializa- 
tion of  an  efficient  starch-making  organ  (figs.  45,  46).  Stomata  are  few 
and  scattered,  on  the  under  side  of  the  leaf  only.  Instead  of  the  character- 
istic palisade  found  in  the  leaves  of  most  other  plants  and  in  the  joints  and 
fruits  of  Opuntia  fulgida  itself,  we  find  the  whole  outer  region,  especially  on 
the  dorsal  side  of  this  leaf,  made  up  of  nearly  isodiametric  cells,  among 
which  are  scattered  small  cells  containing  calcium  oxalate  crystals  and  much 
larger  cells  filled  with  slime  or  mucilage.  The  latter  are  of  the  type  that 
will  be  described  in  more  detail  when  we  come  to  the  consideration  of  the 
internal  structure  of  the  fruit  (figs.  45,  46).  l^ear  the  flattened  base  of  the 
leaf  (fig.  45),  five  vascular  bundles  are  to  be  seen  in  its  cross-section,  but 
only  the  middle  one  of  these  reaches  to  the  tip  of  the  leaf.  The  subordinate, 
lateral  bundles  are  made  up  chiefly  of  short,  broad,  thick-Wtilled  elements 
which  are  often  oriented  transversely  to  the  leaf.  The  principal  (median) 
bundle  also  includes  many  of  these  element-s  at  its  upper  end,  but  has  a 
larger  proportion  of  more  elongated  tracheal  elements  in  its  lower  portion 
(figs.  45,  56). 

AREOLES  OR  AXILLARY  BUDS  OF  THE  OVARY. 

By  far  the  most  significant  peculiarity  in  structure  of  the  wall  of  the  ovary 
in  Opuntia  fulgida,  as  compared  with  other  angiosperms,  is  the  presence  of 
the  axillary  buds  or  areoles  distributed  over  its  surface.  There  is  one  of 
these  within  the  leaf,  or  its  scar,  at  the  top  of  each  tubercle,  though  the  basal 
ones  of  each  fruit  remain  very  rudimentary  and  never,  as  far  as  discovered, 
give  rise  to  any  structures  other  than  a  few  small  spicules  or  an  occasional 
adventitious  root  on  fruits  fallen  to  the  ground.  What  makes  these  axillary 
buds  or  areoles  of  the  fruits  of  prime  significance  in  the  life  of  the  plant  is 
the  fact  that  those  at  least  of  the  upper  two-thirds  of  the  fruit  remain  active 
and  each  capable  of  giving  rise,  on  the  attached  fruit,  to  a  secondary  flower 
or  fruit.  Or,  if  the  fruit  bo  detached  from  the  plant,  these  areoles  may  give 
rise  to  adventitious  roots  and  to  vegetative  shoots,  thus  initiating  new  plants. 


12  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

The  facts  and  structures  of  interest  in  connection  with  these  areoles  are: 
their  number  and  distribution,  the  origin  of  the  growing-point,  the  trich- 
omes,  the  nectaries,  the  spines,  and  the  bristles  or  spicules. 

NUMBER.  SIZE,  AND  DISTRIBUTION  OF  THE  AREOLES. 

Since  there  is  a  bud  in  the  axil  of  each  of  the  leaves  of  the  wall  of  the 
ovarj,  except  in  the  cases  of  4  or  5  of  the  upper  ones  tkat  are  appressed 
against  the  sepals,  the  total  number  of  areoles  formed  is  nearly  the  same  as 
that  of  the  leaves.  Mature  ovaries  show  from  15  to  35  or  (in  joint-fruits) 
even  40  areoles.  But  these  are  not  by  any  means  alike  in  size  or  in.  the 
number  of  organs  or  organ  rudiments  present  in  them.  The  size  varies 
from  1.5  to  3  mm.  in  diameter  in  flowers  just  opening,  while  the  areoles  of  a 
three-year  old  fruit  may  become  4  or  5  mm.  broad  by  6  or  8  mm.  long  ( figs, 
4,  8,  47,  49).  The  areoles  of  the  upper  third  of  the  fruit  are  in  general 
larger  and  more  complexly  organized,  while  those  of  the  lower  third  are 
usually  much  smaller  and  of  very  simple  structure.  The  former  are  the 
ones  most  likely,  under  satisfactory  conditions,  to  develop  further.  The 
lower  ones  usually  grow  little  after  the  maturing  of  the  fruit.  They  gTa du- 
ally become  depressed  more  and  more  deeply  into  the  surface  of  the  fruit, 
till  they  are  nearly  buried  from  sight.  The  lower  half-dozen  areoles  have 
never  been  seen  to  give  rise  to  either  flowers  or  vegetative  branches.  Tn  a 
fallen  fruit,  however,  adventitious  roots  may  push  out  the  upper  border  of 
such  a  dormant  or  apparently  dead  areole  (fig.  100). 

As  seen  from  without,  the  mature  areole  appears  as  a  grayish  yellow, 
bulging  cushion,  of  circular  or  somewhat  longitudinally  elongated  outline 
(figs.  4,  8,  47).  The  surface  of  this  cushion  is  made  up  of  the  ends  of 
hundreds  of  spirally  striated  trichomes,  which  at  first  surround  and  overtop 
all  other  rudiments  in  the  areole.  In  slightly  advanced  areoles  dozens  or 
scores  of  straight,  barbed  bristles  or  glochidia  push  from  beside  or  beneath 
the  tuft  of  trichomes  in  the  apical  half  of  the  areole.  ITear  the  middle  of 
such  a  cushion  may  be  seen  the  flattish  tops  of  one  or  several  spine-tipped 
nectaries  (figs.  14,  47,  48,  49,  50).  Still  later,  in  certain  of  the  areoles  of 
the  upper  third  of  the  ovary,  the  tip  of  the  bud  of  a  secondary  flower  may  be 
seen.  This  is  at  first  covered  by  a  protecting  lattice  made  up  of  the  peg-like 
leaves,  which  push  out  of  the  cushion  of  trichomes,  just  below  the  crescentic 
group  of  spicules  and  just  above  the  nectary  or  nectaries  of  the  areole  (figs. 
13,  15,  47).  In  the  lower  areoles  of  some  of  the  more  elongated  fruits  one 
or  two  spines  may  be  formed  which  resemble  those  found  in  the  areoles  of  the 
vegetative  joints,  but  are  usually  weaker  (figs.  13,  28). 

ORIGIN  OF  THE  GROWING-POINT  OF  THE  AREOLE  AND  THE  ORDER  OF 
INITIATION  OF  ITS  ORGANS. 

The  growing-point  of  the  areole  becomes  distinguishable  at  a  time  when 
the  subtending  leaf  has  attained  less  than  a  quarter  of  its  mature  length; 
that  is,  when  it  is  only  0.5  mm.  long.     It  at  first  consists  of  a  very  small 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  13 

gToiip  of  more  darkly  staining  cells,  located  in  the  very  axil  of  the  yonng 
leaf  (fig's.  12  at  x,  14  at  x\  16).  With  the  further  growth  of  the  base  of 
the  leaf  the  supjiorting  portion  of  the  stem  and  the  tissue  derived  from  the 
axillary  bud  itself  together  push  outward  and  upw-ard  (see  Goebel,  1889,  p. 
79)  to  form  the  young  tubercle  or  mammilla  (hgs.  14,  15,  50).  This  com- 
binati(m  structure  gTOws  more  rapidly  on  the  outer  side,  with  the  result  that 
the  grownng-point  of  the  areole  comes  to  lie  on  the  inner  face  of  the  tubercle 
(fig.  14).  This  shoot  apex  forms  a  slightly  bulging  dome  of  about  half  the 
length  of  the  tubercle  and  facing  directly  toward  the  growing  apex  of  the 
flower  (fig.  14).  Later,  by  the  growth  of  the  tissues  and  organs  arising 
from  the  adaxial  side  of  the  growing-point,  the  upper  end  of  the  tubercle 
becomes  directed  more  outwardly,  often  at  an  angle  of  45°  with  the  axis  of 
the  flower  (figs.  15,  17,  19).  The  growing-point  of  the  areole  from  this 
time  onward  faces  almost  directly  upward  (figs.  12,  17,  20,  24). 

The  first  nidiments  to  appear  on  the  growing-point  of  the  new  axillary 
bud  are  the  monosiphonous  trichomes,  which  arise  on  the  margin  next  the 
leaf.  Following  these  trichomes  there  appears  a  nectary  and  more  trichomes, 
on  tlie  same  side,  and  later  another  series  of  trichomes  on  the  opposite  or 
inner  margin  of  the  growing-point  (figs.  12,  17). 

TRICHOMES  OF  THE  AREOLE. 

The  first  organs  to  be  developed  in  the  areole,  after  the  growing-point 
itself,  are,  as  noted  above,  the  monosiphonous  trichomes.  These  are 
developed  in  large  numbers,  scores  or  hundreds,  by  the  proliferation  of 
many  adjoining  superficial  cells  about  the  gro^^'ing-point.  At  first  they 
appear  aroimd  half  the  circumference  of  the  gro^nng-point  on  the  side  next 
the  subtending  leaf  (figs.  17,  32,  50).  Soon  afterward  others  appear,  in 
smaller  numbers,  on  the  side  of  the  growing-point  next  the  main  axis. 
When  still  later  a  group  of  spicules  appears  on  this  side  of  the  gi-owing-point, 
and  successive  nectaries  on  the  abaxial  side,  both  sorts  of  structures  are  sur- 
rounded, and  more  or  less  hidden,  by  the  masses  of  trichomes  developed 
about  them.  The  youngest  trichomes,  when  3  or  4  cells  long,  are  bent  over 
the  growing-point  (figs.  10,  50).  Later,  w^hen  they  attain  their  mature 
length  of  8  or  10  cells,  they  stand  up  nearly  perpendicularly  about  the 
gromng-point,  though  they  may  (especially  in  the  upper  portion)  become 
considerably  bent  or  kinked  (figs.  50,  51).  The  mature  trichome  consists 
of  a  single  row  of  from  6  to  10  or  12  cells.  It  is  about  10  or  12  microns  in 
diameter  at  the  base  and  three  or  four  times  this  at  the  top.  The  basal  cells 
of  the  trichome  are  usually  cylindrical,  with  thin,  smooth  walls,  while  the 
up^Der  3  or  4  cells  are  often  barrel-shajwd  and  have  thickened,  spirally 
marked  walls  (figs.  51,  52).  The  terminal  cell  is  often  oval,  with  the 
smaller  end  upward.  In  the  older  trichomes  one  or  more  of  the  terminal 
cells  may  have  fallen  off,  leaving  the  hair  with  a  square  end,  commonly  the 
open  end  of  an  empty  dead  cell. 


14  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

There  can  be  no  doubt  that  these  trichomes  serve  to  protect  from  des- 
iccation the  growing-point  of  the  areole  and  the  young  rudiments  formed  by- 
it.  In  other  words,  they  serve  the  function  served  by  bud-scales,  or  modi- 
fied leaves,  in  the  axillary  buds  of  most  woody  plants.  The  number  and 
length  of  these  trichomes  enables  them  completely  to  submerge  all  other 
structures  in  the  bud  except  the  nectaries  and  the  full-grown  spicules.  The 
latter  protrude  for  half  their  length,  while  the  flat  ends  of  the  former  can  be 
seen  in  surface  view  of  the  areole,  each  with  a  densely  packed  ring  of 
trichomes  about  it  that  have  been  crowded  aside  by  the  swelling  of  the 
nectary.  These  clustered  trichomes  make  a  more  eftective  protection  also 
because  of  the  enlarged,  thick-walled  cells  at  the  end  of  each  of  them.  The 
lattice-like  thickening  of  these  cells  enables  them  to  maintain  their  form  and 
full  size  when  dried  out  completely  by  the  desiccating  winds  of  their  native 
habitat.  Whether  the  trichomes  have  other  functions  at  any  other  period 
of  their  existence  has  not  been  detennined.  None  has  suggested  itself  to 
the  writer  as  probable  in  the  course  of  this  study. 

The  final  fate  of  these  trichomes  has  been  suggested  by  what  was  said  of 
the  breaking-off  of  the  terminal  cells.  This  process  is  apparently  repeated 
until  the  trichome  practically  disappears ;  at  least,  the  older  areoles  contain 
large  numbers  of  decapitated  hairs,  many  of  them  with  only  a  few  of  the 
basal  cells  left. 

NECTARIES  OF  THE  AREOLE:  THEIR  DISTRIBUTION  AND  MORPHOLOGY. 

In  every  areole,  soon  after  the  differentiation  of  its  growing-point  and  the 
development  of  a  few  score  of  trichomes,  there  appears  among  the  latter,  on 
the  side  of  the  growing-point  toAvard  the  leaf,  the  rudiment  of  a  nectary. 
In  many  of  the  smaller,  basal  areoles  of  the  fruit  only  one  or  two  of  these 
nectaries  may  be  formed,  and  these  lie  close  to  the  sagittal  plane  of  the 
areole  (figs.  24,  47).  In  the  larger,  upper  areoles,  however,  the  nimiber  of 
nectaries  may  continue  to  increase  with  the  growth  of  the  areole  until,  by  the 
end  of  the  first  gro^ving-season,  there  may  be  8  or  10  nectaries  present  in. 
each  (figs.  5,  47,  48,  49).  As  the  areole  grows  year  after  year  on  the  per- 
sistent fruit,  the  number  of  living  and  withered  nectaries  may  continue  to 
increase  till  20  or  more  have  been  formed.  iRot  more  than  4  or  5  mature, 
living  nectaries  are  present  at  one  time,  but  a  number  of  younger  ones  may 
be  initiated  between  a  mature  one  and  the  growing-point  before  this  begins 
to  shrivel.  In  the  upper  areoles  of  a  primary  ovary  secondary  flower-buds 
usually  appear  after  but  2  or  3  nectaries  have  been  developed,  and  these  are 
soon  crowded  aside  to  wither  as  the  secondary  bud  swells  (figs.  47,  49).  In 
case  secondary  flowers  are  not  formed  from  an  areole  until  the  second  or  a 
later  year,  there  may  be  many  old  nectaries  found  crowded  aside  or  partially 
crushed  by  the  stalk  of  the  enlarging  secondary  fruit  (fig.  14).  These  nec- 
taries were  evidently  initiated  before  the  flower,  since  the  latter,  as  we  have 
seen,  involves  the  whole  gi'owing-point  of  the  areole  (figs.  10,  12,  14). 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  15 

The  youngest  stage  of  tlie  nectary  seen  was  a  low,  conical  projection  of  the 
superficial  layers  of  the  nieristem  among  the  trichomes  immediately  beside 
(abaxial  to)  the  growing-point  of  the  areole  (figs.  12,  50).  Very  soon  a 
small  vnscnlar  strand  is  differentia  tod,  just  helow  the  base  of  the  nectary, 
which  later  penetrates  a  short  distance  into  it  (figs.  50,  5G).  As  the  nec- 
tary grows  it  widens  somewhat  near  the  top  and  becomes  more  or  less  con- 
stricted at  the  base  (fig.  17).  When  mature,  the  stalk  of  the  nectary  has 
alx)ut  three-fifths  the  diameter  of  its  upper  half.  The  top  usually  has  a 
small  depression  with  a  small  tubercle  at  its  center  (figs.  17,  20).  Some- 
times this  tubercle  develops  to  a  well-marked  spine  (fig.  23).  These  facts 
clearly  indicate,  as  has  been  noted  by  Ganong  (1894,  p.  59),  that  the  nectary 
is  homologous  with  the  spines  that  are  so  abundant  on  the  joints  of  many 
opuntias,  but  are  often  wanting  on  their  fruits.  The  steady  increase  in 
number  of  the  nectaries  is  therefore  strictly  comparable  with  the  constant 
increase  in  number  of  the  spines  of  the  areole  of  the  vegetative  joint 
{cf.  p.  8)._ 

The  epidermis  of  the  mature  nectary  is  small-celled  and  thin-walled, 
except  at  the  top  and  on  the  spine  or  tubercle  itself.  Here  the  cells  are  small 
and  irregularly  compacted  and  their  walls  are  provided  ^Ai\\  a  cutin  layer 
several  microns  in  thickness,  which  peels  off  rather  readily  (fig.  56).  The 
cells  of  the  interior  of  the  nectary  seem  to  be  little  differentiated,  except  for 
the  occasional  trace  of  a  vascular  bundle  near  the  base.  They  are  often 
longitudinally  elongated  to  10  or  12  times  their  diameter  and  are  commonly 
pointed  at  both  ends  (fig.  56).  Most  of  these  cells  in  the  mature  nectary 
have  darkly  staining  protoplasts  and  nuclei,  but  a  considerable  number  of 
them  are  nearly  devoid  of  contents  except  for  a  large,  stellate  crystal  of 
calcium  oxalate. 

The  mature  nectary  remains  plump  and  probably  active  for  one  growing 
season  and  then,  as  has  been  suggested,  it  gradually  withers  and  dries  up  to  a 
shriveled  brown  rod  of  scarcely  a  tenth  the  diameter  of  the  functional  nec- 
tary. It  is  this  shriveled  mummy  that  persists  indefinitely,  year  after  year, 
among  the  trichomes  of  the  areole  (fig.  14). 

SPINES  OR  THORNS  OF  THE  AREOLE :    THEIR  DISTRIBUTION,  STRUCTURE.  AND  FATE. 

"\Mnle  spines,  to  the  number  of  6  or  8  and  of  a  length  of  3  or  4  cm.,  are 
present  in  areoles  of  the  vegetative  joint  of  Opuntia  fuJgida,  they  are  usually 
wanting  from  the  fruits  {cf.  Wetterwald,  1889,  fig.  18).  When  spines  do 
occur  in  a  fruit  there  is  usually  but  a  single  spine  in  each  of  only  a  few  of 
its  areoles  (figs.  V),  7c).  The  position  of  this  spine  is  essentially  that  of  the 
first  nectary  of  the  more  normal  fruits ;  that  is,  it  stands  in  the  sagittal  plane 
of  the  areole,  just  wathin  the  subtending  leaf. 

The  structure  of  these  spines  may  be  described  briefly  here,  leaving  the 
fuller  discussion  of  this  and  their  development  for  a  later  paper  in  which  it  is 
planned  to  deal  more  in  detail  with  the  stem.  The  spines  initiated  on  the 
fruit  seldom  attain  the  size  and  strength  of  those  on  the  stem.     They  also 


16  THE   FEUIT   OF   OPUNTIA   FULGIDA. 

often  drop  off  with  the  maturing  of  the  fruit,  apparently  in  consequence  of 
the  withering  of  the  base  of  the  spine.  The  number  of  spines  in  the  spine- 
bearing  areoles  of  the  joint-like  fruits  evidently  increases,  from  year  to  year, 
much  as  in  the  areoles  of  the  stem. 

Each  spine  consists  of  a  slender,  barbed  axis  or  core  and  a  glistening 
white,  striated  sheath.  The  surface  cells  of  the  core,  in  the  upper  half  of  its 
length,  project  outward  and  downward  at  the  tip  to  form  the  extremely  sharp 
retrorse  barbs  (figs.  53,  54).  The  sheath,  which  at  first  covers  the  whole 
core  of  the  spine  with  a  tightly  fitting  jacket,  is  made  up  of  several  layers  of 
greatly  elongated  and  very  thick-walled  cells  (figs.  54,  55),  As  the  spine 
matures  it  shrinks  in  diameter  and  separates  from  the  sheath.  The  latter  at 
the  same  time  contracts  longitudinally,  so  that  its  tip  is  punctured  by  the 
point  of  the  spine.  Later  the  basal  portion  of  the  sheath  splits  to  several 
strips,  which  are  soon  folded  back  on  themselves  in  loops  (fig.  53).  The 
result  of  this  is  that  the  tips,  even  of  spines  developed  in  the  greenhouse,  are 
left  naked  for  several  millimeters. 

BRISTLES  OR  SPICULES  OF  THE  AREOLE:  THEIR  NUMBER.  STRUCTURE.  AND  FATE. 

On  the  inner  or  abaxial  margin  of  each  areole  there  is  a  crescentic  group 
of  barbed,  yellow,  weak-based  bristles,  which  form  the  only  armament  of 
most  fruits  of  this  species.  This  curved  cluster  of  spicules  reaches  about 
one-third  way  around  the  growing-point  of  the  areole  (figs.  32,  50).  The 
tips  of  the  older  bristles  lie  close  against  the  surface  of  the  ovary  just  above 
the  areole.  Each  individual  bristle  is  practically  straight,  about  50  or  60 
microns  in  diameter  and  1  to  1.5  mm.  long.  The  surface  of  the  bristle  is 
made  up  of  thick,  yellow-walled  cells  5  to  8  microns  broad  by  100  microns 
long.  The  outer  ends  of  these  cells  project  slightly  outward  and  sharply 
downward  to  form  the  characteristic  barbs  which  make  these  bristles  such  a 
persistent  and  irritating  reminder  of  an  encounter  with  the  fruits  or  joints 
of  this  cactus  (figs.  57,  58).  The  cells  of  the  interior  of  the  bristles  are  of 
slightly  smaller  diameter,  more  elongated,  with  clear  and  much  thinner  walls 
(figs.  58,  59). 

The  bristles  are  the  last  of  the  several  types  of  organs  to  appear  in  the 
areole.  Even  the  first  of  them  do  not  appear  till  scores  of  trichomes  and 
one  or  two  nectaries  have  been  developed  (figs.  12,  50).  The  rudiment  of 
the  bristle  is  not,  like  that  of  the  trichome,  of  a  single  row  of  cells,  but  is  5  or 
6  cells  across  when  it  first  pushes  out  from  the  growing-point  (fig.  56). 
The  number  of  bristles  in  an  areole  increases  with  age.  At  the  time  the  leaf 
is  shed  the  crescentic  cluster  about  the  growing-point  may  consist  of  6  or  8 
concentric  rows  of  20  to  30  bristles  in  each  row  (figs,  32,  50),  In  the 
sterile  areole  of  a  four-year-old  fruit  this  number  may  be  double  or  triple 
that  just  mentioned.  When  once  formed  the  bristles  evidently  persist  indefi- 
nitely unless  dislodged  by  browsing  animals  or  by  the  development  of  a 
flower  or  shoot  from  the  areole.  The  absorption  of  a  growing-point  in  the 
production  of  such  a  flower  or  shoot  of  course  puts  a  stop  to  the  appearance  of 
further  bristles  from  that  areole. 


l^stjih  Carollna^t|fe?0i'a'y 


THE  FRUIT   OF   OPUNTIA  FULGIDA.  17 

PERIANTH.  STAMENS.  STYLE.  AND  STIGMA:    THEIR  DEVELOPMENT 
AND  ABSCISSION. 

The  order  of  initiation  of  Uie  organs  of  the  flower  is  an  acropetal  one. 
The  series  begins  \ntli  the  fonnation  of  the  peg-like  leaves,  followed  by  that 
of  the  areoles,  the  sepals,  and  petals,  all  from  the  characteristic  convex 
groA\'ing-point  like  that  of  the  stem.  Then  with  a  change  in  the  growing- 
point  to  a  concave,  ciip-like  shape,  tlie  series  of  floral  parts  is  completed  with 
the  initiation  of  the  stamens  and  carpels,  or  perhaps  we  should  say  with  that 
of  the  placentas  and  o^alles,  which  are  fonned  deep  in  the  bottom  of  the  cup 
(cf.  alsoGoebel,  1886). 

The  perianth  of  Opuniia  fulgida  consists  of  about  8  sepals,  light  green  in 
color,  and  of  a  like  number  of  petals,  rose-pink  in  color.  These  sepals  and 
petals  are  initiated  about  the  gro^\'ing-point  in  the  same  way  that  the  leaves 
of  the  ovary  are,  but  differ  from  the  latter  in  the  important  particular  that  no 
axillary  buds  are  developed  at  the  bases  of  the  perianth  members  and  that 
there  is  no  tubercle  formed  at  the  base  of  either  sepal  or  petal  (figs.  17,  22, 
4Y).  In  mature  structure  also  the  perianth  divisions  differ  strikingly  from 
mature  leaves.  Even  the  sepals  are  considerably  broader  and  flatter  than 
the  leaves,  with  more  vascular  bundles,  while  the  obovate  petals  are  very 
broad  and  have  a  far  more  complex,  reticulate  vascular  system  than  the 
leaves  (figs.  47,  64).  The  mature  perianth  opens  after  midday  (in  mid- 
afternoon  according  to  Lloyd,  1907).  It  forms  a  saucer-shaped  flower  an 
inch  or  more  across.  A  few  days  after  opening  the  whole  perianth  falls 
off,  set  free  by  the  fonnation  of  a  well-defined  abscission  layer. 

The  250  stamens  of  the  flower  have  filaments  about  2  or  3  times  as  long 
as  the  anthers.  Each  stamen  arises  as  a  dome-like  elevation,  6  or  7  cells  in 
diameter,  on  the  margin  of  the  now  concave  gTOwing-point  (figs.  16,  18). 
As  the  stamens  develop  they  bend  inward  over  the  growing-point,  the 
youngest  ones  standing  nearly  at  right  angles  t^  the  axis  of  the  flower  (figs. 
17,  20).  Later  they  swell  at  the  end,  as  the  microsporangia  appear,  and 
gradually  become  more  erect,  but  not  completely  so  until  the  flower  is  open 
(figs.  22,  23,  61). 

The  internal  development  of  the  microsporangia  is  apparently  not  essen- 
tially different  from  that  of  the  typical  angiosperm.  The  u}>per,  older 
stamens  open  first,  as  they  dry  out  first.  The  pollen-gi'ains  are  irregularly 
globular,  with  a  yellowish,  pitted  exine  of  about  4  or  5  microns  in  thickness, 
and  of  a  columnar  or  palisade-like  sti'ucture  when  seen  in  optical  section. 

The  carpels,  the  last  structures  of  the  flower  to  be  formed,  are  6  or  some- 
times 7  in  number.  This  is  clearly  indicated  by  the  number  of  nidiments  of 
carpels  initiated  around  the  growing-point,  by  the  number  of  lobes  of  the 
stig-ma,  and  by  the  number  of  placentas  in  the  mature  ovary  (figs.  17,  31, 
32,  33,  34).  No  case  was  observed  with  5  carpels,  the  number  found  by 
Engelmann  ( 1887)  in  the  plants  studied  by  him.  The  first  rudiments  of  the 
carpels  become  evident  after  about  6  or  7  tiers  of  stamens  have  been  devel- 
2 


18  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

oped ;  that  is,  when  there  are  6  or  7  stamens,  one  above  the  other,  on  one  side 
of  the  growing-point  in  a  single  radial  longitudinal  section  (fig.  18).  The 
carpel  rudiment  at  its  initiation  is  twice  as  thick  as  that  of  a  stamen.  It 
differs,  also,  in  that  it  almost  immediately  bends  inward  above  the  growing- 
point  to  meet  its  fellows  and  thus  to  complete  the  roof  of  the  ovarian  cavity 
(figs.  IT,  18,  19).  Soon  after  the  tips  of  the  carpels  meet  they  begin  to  fuse 
together  along  their  radial  surfaces  to  form  the  rather  stout  style,  which 
incloses  a  papilla-lined  stylar  canal,  that  is  star-shaped  in  cross-section  (figs. 
21,  23,  35,  37).  The  very  tips  of  the  carpels,  for  a  length  of  3  or  4  times  their 
diameter,  remain  unfused  and  form  the  stigmatic  lobes.  These  finally 
become  1  to  1.5  mm.  long  and  are  densely  clothed  with  swollen,  sac-like 
hairs  that  serve  for  the  attachment  of  pollen-grains  (figs.  23,  36).  Begin- 
ning at  a  depth  of  5  or  6  layers  inward  from  the  wall  of  the  stylar  canal  is  a 
corrugated  tube  of  conducting  tissue  8  to  20  or  even  30  cells  in  thickness, 
through  which  the  pollen-tube  is  to  push  its  way  (figs.  22,  34,  35).  The 
slender  cells  of  this  conducting  layer  extend  upward  to  the  very  base  of  the 
hairs  of  the  pollen-receiving  surface  of  the  stigma  (fig.  36).  At  the  base  of 
the  style  this  layer  is  continued  downward  as  a  series  of  strands  reaching  to 
the  roof  of  the  ovary  (figs.  22,  23,  24).  The  mature  stigma  lobes  are  con- 
tinued outward  and  downward  to  form  the  characteristic  6-rayed  or  7-rayed 
structure  seen  in  the  open  flower  (figs.  30,  31).  The  lining  of  this  part 
of  the  ovary  wall,  down  to  the  uppermost  ovules,  is  covered  by  slender  hairs 
protruding  into  the  cavity  of  the  ovary.  These  probably  help  to  conduct  the 
pollen-tubes  to  the  micropyles. 

Wot  only  do  the  carpellary  lobes,  which  are  at  first  transverse,  grow 
upward  after  meeting  above  the  depressed  growing-point,  but  they  may 
also  often  grow  downward  somewhat  into  the  ovarian  cavity,  thus  making 
the  roof  of  the  latter  lowest  near  the  center  (figs.  11,  21,  61).  Transverse 
sections  of  the  ovary  at  this  time  may  show  several  upward  prolongations  of 
the  ovarian  cavity,  separated  from  each  other  by  the  downward  growth  of 
the  carpels  at  the  plane  of  juncture  of  the  two  carpels  of  each  pair.  The 
impression  given  by  such  a  section  is  that  of  a  compound  ovary  with  6  or  7 
separate  cavities. 

The  separation  and  fall  of  the  perianth,  and  other  parts  that  fall  with  it, 
is  a  complicated  and  rather  variable  process,  as  compared  with  the  shedding 
of  parts  in  most  choripetalous  flowers.  In  about  3  days  after  the  flower  has 
opened  the  withering  of  the  sepals  and  petals  has  gone  so  far  that  all  those 
of  a  flower  are  twisted  together  into  a  cone  of  dry,  crisp  remnants ;  that  is, 
the  parts  of  the  flower  do  not  drop  off  individually,  as  usually  happens  in 
choripetalous  angiosperms,  but  the  whole  series  of  sepals,  petals,  stamens, 
and  in  some  cases  even  the  style,  are  cast  off  from  the  ovary  at  once,  all 
attached  to  a  cup-like  common  base  stripped  off  from  the  upper  end  of  the 
ovary.  This  wholesale  shedding  of  the  floral  parts  is  accomplished  by  the 
formation  of  a  highly  developed  abscission  layer  across  the  entire  upper  end 


THE   FRUIT   OF   OPUXTIA   FULGIDA.  19 

of  the  ovary.  The  foi-m  of  this  Layer  is  not  a  simple  transverse  plane,  but  is 
that  of  an  inverted  cone,  usually  perforated  at  the  apex.  This  funnel-shaped 
layer  of  tissue  is  initiated  in  cells  15  or  20  layers  beneath  the  surface  of  the 
cup-like  upper  end  of  the  ovary  that  bears  the  stamens,  petals,  and  sepals 
(figs.  23,  58,  60,  61). 

In  a  diametric  longitudinal  section  of  the  ovary  this  abscission  layer 
usually  starts  in  at  the  base  of  perianth,  either  outside  the  sepals  or,  more 
rarely,  between  these  and  the  petals.  From  here  it  extends  downward, 
parallel  to  the  surface  of  the  cup  at  the  top  of  the  ovar^^,  to  a  level  just  a 
little  above  the  base  of  the  style,  where  the  abscission  layer  again  comes  out 
to  the  surface  of  this  cup  (figs.  23,  24,  GO,  68).  Less  frequently  the  more  or 
less  developed  abscission  layer  may  extend  across  beneath  the  bottom  of  the 
cup  at  the  top  of  the  ovary.  In  this  latter  case  the  style  is  cut  off,  along  with 
the  stamens  and  perianth  attached  to  the  same  complete,  shriveled  funnel 
(fig.  61).  In  the  more  usual  case,  first  noted,  the  perianth  and  stamens  only 
are  borne  on  a  shriveled  funnel  that  is  perforated  at  the  base,  while  the  style 
is  shed  separately,  breaking  across  just  above  its  base  (fig.  24).  In  rarer 
cases,  where  the  abscission  layer  starts  in  at  the  top  within  the  perianth  (fig. 
23),  the  i-)etals  and  sepals  must  evidently  be  cut  off  separately.  Whether  a 
real  abscission  layer  is  developed  in  each  part  or  not  was  not  determined. 

The  first  origin  of  the  abscission  layer  across  the  top  of  the  ovary  is  evi- 
denced by  a  swelling,  chiefly  a  radial  elongation,  of  a  continuous  layer  of 
cells  in  the  midst  of  the  wall  of  the  ovary,  in  the  region  stretching  between 
the  base  of  the  perianth  and  the  base  of  the  style  (figs.  61,  68).  Apparently 
any  cells  along  the  line  of  the  abscission  layer  to  be  may  take  part  in  its 
formation,  except  such  specialized  cells  as  those  of  the  vascular  bundle,  the 
mucilage  cells,  and  the  crystal-holding  cells.  The  cells  that  are  to  form 
the  abscission  layer  increase  in  radial  length  to  about  twice  their  tangential 
diameter.  Then  the  cells  divide  tangentially  into  two  nearly  isodiametric 
cells  (figs.  67,  68).  A  further  tangential  division  follows  very  soon  in  each 
of  these  cells,  resulting  in  the  formation  of  a  row  of  4  cells,  of  which  the  two 
middle  ones  have  only  half  the  radial  thickness  of  the  two  outer  (fig.  67). 
It  is  at  this  stage,  or  sometimes  after  one  or  two  further  tangential  divisions, 
that  abscission  occurs.  The  details  of  possible  changes  in  the  radial  walls  of 
tlie  abscission  cells  have  not  been  studied.  (See  Lloyd,  1914,  p.  70,  and 
1916,  pp.  213-230).  It  is  clear,  however,  that  in  consequence  of  the  shrink- 
ing of  the  perianth,  on  drying,  the  delicate  radial  walls  of  the  thin,  tubular, 
cambium-like  cells  in  the  middle  of  the  abscission  layer  are  niptured.  The 
separation  occurs  first  at  the  base  of  the  perianth  and  continues  do-v\mward 
until  the  whole  top  of  the  ovary,  bearing  perianth  and  stamens,  and  clasping 
the  style  within,  curls  together  and  droj)s  ofi^  the  flower  on  the  second  or  third 
day  after  its  opening.  The  surface  left  at  the  top  of  the  ovary  after  the 
shedding  of  the  periantli  shows  cleaidy  that  the  break  tiikes  place  in  one  of 
the  thin  cells  in  the  middle  of  the  abscission  layer,  and  also  that  it  occurs 
in  the  radial  wall  rather  than  as  a  split  between  two  tangential  walls  (fig. 


20  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

62  ) .  Whether  or  not  there  is  a  preliminary  softening  of  the  cell-walls  of  the 
abscission  layer,  there  is  evidently  a  change  in  the  cell-walls  of  the  vascular 
bundles  in  the  line  of  fission,  as  these  become  flabby  and  distorted  before 
abscission  occurs  (fig.  68).  Mucilage  cells  lying  in  the  line  of  fission  are 
usually  not  traversed  by  it,  but  cells  within  or  without  these  become  trans- 
formed into  abscission  cells  (figs.  61,  68). 

The  lining  of  the  funnel  left  at  the  toj)  of  the  ovary  after  abscission  is  thus 
made  up,  from  the  outer  edge  of  the  funnel  do^vn  nearly  to  its  bottom,  of  the 
thin-walled  cells  derived  from  the  abscission  layer.  Immediately  after 
abscission  the  outermost  cells  dry  and  shrivel  (fig.  62),  while  cells  just  below 
the  surface  begin  the  production  of  the  protective  layer  of  cork  cells  which 
will  be  noted  in  describing  the  fruit  (fig.  63).  The  very  bottom  of  the 
funnel  left  after  abscission  is  in  most  cases  formed  by  the  short  stump  of  the 
style,  which  is  apparently  cut  off  by  an  irregmlar  transverse  rupture  of  the 
cells  without  the  formation  of  any  distinct  abscission  layer.  The  wall  of  the 
funnel  for  a  millimeter  just  above  the  stump  of  the  style  is  still  lined  by  the 
epidermis  (fig.  24).  Only  in  those  cases  where  the  abscission  layer  cuts 
across  below  the  base  of  the  style  (fig.  61)  is  the  corky  lining  of  the  funnel 
complete  from  the  start. 

MORPHOLOGY  OF  THE  OVARY. 

The  flower  of  this  species  of  Opuntia,  like  that  of  any  angiosperm,  is  to  be 
regarded  as  a  branch  of  the  shoot  which  bears  carpels,  stamens,  petals,  etc., 
instead  of  the  usual  photosynthetic  leaves,  but  it  is  unusual,  or  indeed  almost 
unique,  in  retaining  a  large  series  of  the  characteristics  of  the  vegetative 
shoot.  This  is  indicated  clearly  by  certain  features  of  its  earlier  develop- 
ment, by  the  occurrence  on  it  of  photosynthetic  leaves  with  axillary  buds,  and 
by  the  occasional  presence  on  it  of  spines,  like  those  of  the  vegetative  shoot. 
Further  evidence  tending  in  the  same  direction  is  offered  by  the  persistence 
and  secondary  growth  of  the  fruit,  by  the  vegetative  multiplication  of  the 
flower  and  fruit,  and,  finally,  by  the  occurrence  of  many  types  of  structures 
intermediate  between  the  tjq^ical  fruit  and  the  typical  vegetative  joint.  In 
this  assemblage  of  peculiarities  the  flowers  of  this  and  certdiin  allied 
opuntias  are  unique,  so  far  as  I  can  learn  from  published  records.  In  the 
allied  genus  Peireskia  the  primary  flower  does,  it  is  true,  bear  several  pairs 
of  green  leaves,  "udth  buds  in  their  axils,  and  2  or  3  of  these  may  give  rise  to 
secondary  flowers.  Tertiary  flowers,  however,  are  more  rare,  as  far  as  I 
have  learned,  and  clusters  of  more  than  3  or  4  mature  fruits  have  not  been 
seen.  ISTo  record  has  been  found  of  these  axillary  buds  of  the  ovary  giving 
rise  (either  before  or  after  the  separation  of  the  fruits  from  the  plant)  to 
vegetative  shoots  of  the  sort  formed  by  fruits  of  Opuntia  fulgida.  In  the 
genus  Cereus  also  something  of  the  same  kind  evidently  occurs  at  times. 
Thus  Harris  (1905,  p.  535)  describes  briefly  the  occurrence,  in  a  specimen 
of  Cereus  hoxaniensis,  of  "  several  teraiinal  fruits,  one  of  which  had  other 
flowers  developing  from  the  side."     A  section  of  one  of  the  (primary  ?) 


THE  FRUIT   OF   OPUXTIA   FULGIDA.  21 

fruits  showed  it  to  be  sterile.  No  information  is  given  conceniing  the 
presence  of  axillary  buds  on  the  primary  fruit,  though  it  is  clear  that  these 
must  have  been  present  to  initiate  the  secondary  flowers. 

The  nearest  approach  to  this  structure  of  the  wall  of  the  ovary  in  these 
CactacesB  of  which  I  find  record  in  any  other  family  is  seen  in  the  genus 
Calycanthus.  In  this  form  the  greatly  developed  floral  axis  is  depressed  at 
the  top  to  form  a  cup,  from  around  the  edges  of  which  arise  the  perianth 
members  and  the  stamens.  The  numerous  distinct  carpels,  on  the  contrary, 
are  developed  from  the  bottom  of  this  cup  and  remain  surrounded  by  it,  but 
do  not  play  any  part  in  roofing  it  over,  as  the  carpels  of  the  Opuntia  flower 
close  above  its  concave  growing-point.  The  outer  wall  of  the  cup  of  this 
fruit  of  Calycanthus  bears  the  scars  of  many  fallen  petaloid  floral  leaves, 
distributed  much  as  the  leaf-scars  and  areoles  are  over  the  fniit  of  Opuntia 
fidgida.  A  study  of  the  development  of  the  flower  of  Calycanthus  shows, 
however,  that  there  are  no  axillary  buds  above  its  leaves.  There  is  not  even 
the  smallest  recognizable  rudiment  of  this,  and  hence  no  possibility  of  the 
development  of  secondary  flowers  or  shoots  from  the  wall  of  the  flower  or 
fruit. 

The  facts  of  development  and  structure  suggested  in  the  paragraph  before 
the  last,  together  with  those  detailed  earlier  in  this  paper,  furnish  important 
if  not  conclusive  evidence  regarding  the  morphological  nature  of  the  ovary  of 
this  opuntia.  To  the  writer  these  facts  seem  to  offer  very  strong  evidence 
for  the  view  that  the  flower  of  the  opuntias  consists  of  a  shorter  or  longer 
vegetative  joint,  into  the  depressed  upper  end  of  which  the  ovary  is  com- 
pletely submerged,  and  around  the  margin  of  which  the  stamens  and 
perianth  members  are  inserted.  This  view  has  been  advanced  in  more  or 
less  definite  form  by  various  workers  on  the  Cactacea;  in  the  past.  (See 
Schumann,  1894,  p.  168;  Zuccarini,  1844;  Toumey,  1905,  p.  235;  Harris, 
1905,  etvC.)  The  evidence  for  this  view,  however,  has  hitherto  always 
seemed  somewhat  inadequate.  Since  it  is  felt  that  the  present  study  offers 
the  most  complete  chain  of  evidence  thus  far  produced,  especially  from  the 
developmental  standpoint,  this  evidence  will  be  stated  here  in  some  detail. 

In  the  first  place  the  external  structure  of  the  ovary  at  the  time  of  opening 
of  the  flower  is  very  like  that  of  a  vegetative  joint,  having  prominent  mam- 
milla?, each  of  them  bearing  a  conical  leaf  and  a  bud  in  the  axil  of  the  latter, 
and  (rather  rarely)  a  spine  like  those  of  the  stem  itself.  As  stated  above, 
the  ovaries,  and  the  fruits  formed  from  them,  differ  markedly  in  length  and 
breadth.  Thus,  the  basal  part  of  the  ovary  may  sometimes  be  short,  witli 
relatively  few  mammilla?  and  areola^,  and  so  give  rise  to  a  nearly  globular 
fruit  in  which  the  ovarian  cavity  reaches  nearly  to  tlie  base  (figs.  11,  24,  26, 
27).  In  other  cases  the  basal  portion  of  the  ovary  may  be  far  more 
developed,  with  20  or  even  30  tubercles,  and  the  ovarian  cavity  may  occupy 
only  the  upper  third  or  fifth,  or  even  less,  of  the  portion  of  the  whole  floral 
joint  below  the  perianth  (figs,  lb,  28).     lu  these  extremely  long  flowers  the 


22  THE   FEUIT   OF   OPUNTIA   FULGIDA. 

surface  of  tJie  ovary  at  the  base  may  be  much  more  stem-like  than  at  its  upper 
end,  having  more  prominent  tubercles  and  furnished  with  areolae  that  not 
infrequently  bear  a  spine  or  two  each.  ISTot  only  do  the  real  functional 
flowers  and  f iiiits  differ  considerably  in  size  and  in  the  relative  development 
of  parts,  but  the  semblance  to  the  stem  may  go  so  far  that  the  perianth  fails 
to  open,  or  even  fails  to  develop  completely,  or  (in  extreme  cases)  the  carpels 
or  ovules  or  even  the  ovarian  cavity  itself  may  never  be  initiated  at  all. 

Perhaps  the  small,  smooth  joiutlets  or  pseudo-fruits  mentioned  by 
Toumey  (1905,  p.  531)  are  to  be  regarded  as  the  final  members  of  this  series 
of  simplified  fruits.  These  pseudo-fruits  occur  in  more  or  less  dense,  fruit- 
like clusters,  which  Toumey  says  are  particularly  abundant  in  adverse  sea- 
sons, when  true  fruits  are  less  abundant.  Such  structures  resemble  true 
fruits  in  the  lack  of  spines  and  in  the  less  marked  tubercles,  but  differ  in 
having  no  trace  of  a  flower  or  a  perianth  scar  at  the  terminal  end. 

These  joints  (intermediate  in  character  between  flowers  or  fruits  and 
normal  vegetative  joints)  occur  still  more  frequently  in  the  allied  species 
Opuntia  leptocaulis,  in  which  practically  all  degrees  of  reduction  of  the 
floral  parts  can  readily  be  found.  Perhaps  as  complete  a  series  could  be 
found  for  0.  fulgida  after  long  search,  but  the  intermediate  types  are  far 
more  abundant  in  0.  leptocaulis  (figs.  89,  96).  In  the  platopuntias  also 
joint-fruits  quite  similar  in  character  to  those  of  0.  fulgida  are  not  really 
infrequent  where  large  numbers  of  these  plants  can  be  examined  in  the  field. 
The  stem-like  character  of  these  joint-fruits  is  still  more  clearly  indicated  in 
these  forms,  because  the  normal  joints  are  flat,  while  the  fruits  are  usually 
barrel-shaped  or  obconcial.  A  nimiber  of  examples  of  these  unusual  fruits 
of  the  flat  opuntias  have  been  mentioned  in  the  literature,  chiefly  on  plants 
growing  in  greenhouses.  Most  of  these  abnormal  structures  have  rather 
typical  perianths,  sporophylls,  ovarian  cavities,  etc.,  but  the  wall  of  the 
ovary,  instead  of  being  as  usual  a  radially  symmetrical  structure,  grows  out 
on  two  sides  to  wing-like  expansions.  This  gives  the  whole  structure  the 
appearance  of  a  disk-like  vegetative  joint  with  a  thicker  ovary  embedded  in 
its  upper  margin.  This  ovary  may  be  at  the  very  top  or  down  more  or  less 
on  the  lateral  margin  of  the  disk. 

Secondly,  the  intenial  structure  also  of  the  ovary  of  Opuntia  fulgida  is 
essentially  like  that  of  the  vegetative  joint.  Thus  the  organization  of  the 
areole,  the  photosynthetic  system,  and  the  vascular-bundle  system  are  alike 
in  the  two  structures,  except  for  the  additional  vascular  branches  supplying 
the  perianth  and  sporophylls  of  the  flower  (fig.  28). 

A  third  feature  in  which  the  fruit  of  Opuntia  fulgida  resembles  the  vege- 
tative shoots  of  the  same  plant  is  in  its  ability  to  persist  for  some  years  on 
the  parent  plant  and  to  continue  to  grow  in  thickness  year  after  year  by  a 
well-defined  cambium  layer.  This  point  is  dealt  with  in  more  detail  else- 
where (see  p.  29). 

Fourthly,  the  capacity  mentioned  elsewhere  for  self-propagation  by  the 
attached  flower  and  that  of  shoot-production  by  the  fallen  fruit  are  still 


THE  FRUIT   OF   OPUNTIA   FULGIDA.  23 

other  features  in  which  these  structures  resenihlo  the  v('<;ctativt'.  joints  of  the 
shoot. 

Fifthly  and  lastly,  the  whole  history  of  development  of  the  flower  and 
fruit,  when  compared  with  that  of  the  vegetative  shoot,  shows  clearly  that  the 
whole  lower  portion  of  the  ovary,  up  to  the  perianth,  is  developed  by  a  convex 
growing-point,  in  every  respect  just  as  a  branch  of  the  stem  is.  It  is  evident 
that  if  the  growing-point  of  a  flower,  when  once  initiated,  continues  its  activ- 
ity but  a  short  time  before  the  members  of  the  perianth  are  laid  down,  the 
ovary  of  that  flower  will  have  only  a  short  vegetative  portion,  and  the  ovarian 
cavity  will  occupy  a  considerable  portion  of  the  whole  length  of  the  ovary 
(figs.  23,  25).  If,  on  the  contrary,  the  growing-point  that  is  to  form  a 
flower  continues  longer  to  form  leaves,  areoles,  tubercles,  etc.,  there  may  be  a 
long  stretch  of  vegetative  axis  developed  Ijefore  the  perianth  and  sporophylls 
are  initiated.  Thus  the  ovary  proper  may  occupy  but  a  small  portion  of  the 
upper  end  of  the  whole  segment  or  joint  which  is  the  product  of  the  continu- 
ous activity  of  this  individual  gromng-point  (figs.  7b,  1c,  28). 

The  comparison  of  figures  17,  19,  22,  and  23  shows  clearly  that  the  ovary 
of  this  cactus  is  really  secondarily  submerged  by  the  upward  and  inward 
growth  of  the  portions  of  the  axis  just  below  the  perianth.  The  result  of 
this  growth  is  that  the  ovarian  cavity,  at  first  formed  on  a  level  with  the 
uppermost  areoles  (figs.  16,  17),  is  buried  more  and  more  deeply  by  this 
growing  upward  and  rolling  inward  of  the  parts  of  the  wall  of  the  ovary  that 
were  laid  do^vn  before  the  cavity  of  the  ovary  had  appeared.  This  ditfer- 
ential  growth,  which,  during  the  ontogeny  of  the  organ,  buries  the  originally 
nearly  superficial  and  terminal  ovarian  cavity,  until  it  lies  near  the  middle 
in  the  older  flower  and  fruit,  suggests  clearly  the  probable  phylogenetic 
origin  of  the  type  of  fruit  found  in  this  opuntia. 

A  very  striking  feature  of  shoot  development  in  these  opuntias  is  the 
marked  constriction  usually  formed  at  the  limits  marking  the  growths  of  the 
same  growing-point  in  successive  seasons.  This  corresponds,  of  course,  to  the 
boundaries  marked  by  the  winter  bud-scale  scars  on  the  shoots  of  woody 
dicotyledons.  The  anatomical  reasons  for  this  constriction  in  these  opuntias 
are  not  entirely  clear.  Apparently  the  protective  structures  formed  about 
the  terminal  bud  of  the  annual  shoot  at  the  end  of  the  growing-season  render 
the  tissue  here  firm  and  incapable  of  swelling  out  in  the  follo^ving  spring  to 
the  thickness  of  the  middle  portion  of  this  shoot. 

The  characteristic  origin  of  the  flower  in  the  cylindropuntias  is  also,  as 
we  have  seen,  from  a  new  axillary  bud  which  develops  a  flower  with  its  longer 
or  shorter  ovary  in  the  same  season  that  it  pushes  out  of  the  areole.  In  cer- 
tain rare  instances  in  Opuntia  fulgida,  0.  spinosior,  and  in  others  seen  in 
0.  cylindrica,  growing  at  Del  Monte,  California,  the  part  of  a  flower-forming 
joint,  containing  the  ovarian  cavity,  was  marked  off  from  the  basal  portion 
only  l)y  a  veiy  slight  narrowing  of  the  joint.  Such  structures  seem  to  indi- 
cate that  the  same  growing-point  may  sometimes  develop  continuously  either 


24  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

during  the  same  season,  or  more  probably  for  two  successive  seasons,  forming 
a  vegetative  axis  in  the  first  and  a  flower  and  fruit  in  the  second.  This  at 
least  is  the  most  plausible  explanation  that  suggests  itself  of  the  origin  of 
such  structures  as  those  shown  in  figures  7c  and  88,  where  nothing  but  a  very 
slight  constriction  separates  a  well-developed  fruit  from  a  vegetative  joint  of 
the  usual  length  of  a  yearns  shoot. 

This  same  sort  of  change  in  the  nature  of  the  product  of  a  growing-point, 
either  between  the  beginning  and  end  of  the  same  growing-season  or  in  suc- 
cessive seasons,  will  probably  prove  to  be  the  explanation  of  the  origin  of 
certain  combination  joint-fruits  found  in  the  flat-jointed  opuntias. 

The  most  interesting  problem  concerning  the  development  of  joint  and 
fruit  in  these  cacti  is,  of  course,  that  of  the  cause  determining  that,  up  to  a 
certain  stage  in  the  history  of  the  gromng-point  of  each  flower-rudiment, 
there  shall  be  formed  photosynthetic  leaves,  tubercles,  and  areoles  only,  while 
beyond  this  point  the  course  of  development  is  so  changed  that  thereafter 
nothing  but  floral  structures  are  laid  down  about  the  margin  of  this  identical 
group  of  initials.  The  fact  that  this  change  in  the  character  of  the  rudi- 
ments produced  on  the  growing-point  occurs  at  different  times  in  different 
flowers  of  the  same  plant  seems  to  indicate  that  the  conditions  controlling 
this  are  somewhat  local  in  nature.  Such  experimental  attempts  as  were 
made  to  change  the  fate  of  the  structures  organized  about  the  growing-points 
of  very  young  flowers,  by  removing  the  persistent  fruits  bearing  these  young 
flower-buds,  gave  no  clue  to  the  cause  or  nature  of  the  change  in  character  of 
these  rudiments.  These  experiments  did,  however,  show  that  when  the 
change  has  once  occurred  the  character  and  fate  of  these  buds  of  the  areoles 
are  not  reversible.  In  other  words,  if  a  fruit  of  Opuntia  fulgida  is  plucked 
in  February  or  March  and  placed  in  moist  sand,  certain  of  its  areolesi 
will  give  rise  to  vegetative  shoots.  If,  on  the  contrary,  the  fruits  were  picked 
and  planted  in  April,  after  the  floral  structures  have  been  initiated,  these 
same  areoles  wither  without  giving  rise  to  any  permanent  structures  {i.  e., 
shoot-buds)  such  as  would  have  been  formed  by  these  very  same  areoles  if 
picked  a  few  weeks  earlier.  Further  experiments  are  being  undertaken  in 
the  hope  of  discovering  the  material  or  responsive  basis  of  this  change. 

It  would  seem,  then,  that  we  have  strong  evidence,  from  its  structure  and 
development,  for  believing  that  the  present  type  of  fruit  in  Opuntia  has 
arisen  from  an  originally  superior  ovary  which  has  progressively  sunken 
more  and  more  into  the  upper  end  of  the  joint  bearing  it.  However,  as 
Harris  (1905)  has  pointed  out,  there  is  as  yet  no  adequate  evidence  for  con- 
cluding with  Toumey  (1905)  that  this  submergence  has  occurred  very 
recently  in  the  phylogeny  of  the  genus  or  family.  It  is  still  possible,  of 
pourse,  that  Opuntia  is  a  less-modified  type  of  a  series  of  which  Cerev^, 
Echinocereus,  and  Echinocadus  are  more  highly  evolved  members.  In  the 
latter  genera  the  wall  of  the  ovary  may  bear  numerous  bract-like  leaves 
resembling  those  of  the  opuntias  and  often  also  bear  axillary  areoles,  having 
more  or  less  abundant  trichomes  and  in  some  cases  one  or  several  spines. 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  25 

THE  FRUIT:   ITS  STRUCTURE.  PERSISTENCE.  AND  FATE. 
NORMAL  AND  ABNORMAL. 

The  fruit  of  Opuntia  fidgida  occurs,  as  we  have  seen,  in  ehisters  of  from  a 
dozen  to  a  hundred  or  more  depending  from  one  vegetative  joint  or  even,  from 
a  single  parent  frnit  (figs.  2,  3).  The  individual  fruit  may  he  globular  in 
form,  or  barrel-shaped,  pear-shaped,  or  still  more  elongated  and  nearly  cylin- 
drical (figs.  3,  4,  7,  26,  28).  It  may  have  a  diameter  of  from  20  to  35  mm. 
and  a  length  of  from  25  to  65  mm.  Its  size  differs  with  the  plant  and  with 
its  position  in  the  cluster.  The  terminal  and  younger  fruits  are  usually 
smaller,  but  sometimes  those  of  any  one  growing-season  seem  small  through- 
out. The  surface  of  the  upper  or  terminal  end  of  the  fruit  is  formed  by  the 
cork-covered  scar  left  by  the  fall  of  the  perianth  and  stamens.  This  scar  is 
decidedly  concave  when  young,  but  with  age  it  flattens  out  or,  in  some 
cases,  even  bulges  slightly  at  the  center  (figs.  3,  25,  28).  The  lateral  sur- 
face of  the  fruit  when  young  has  markedly  developed  tubercles,  or  mammil- 
lae, each  terminating  in  a  leaf-scar  and  its  accompanying  areole  (figs.  4,  17, 
31,  32,  47).  These  tubercles  become  less  prominent  as  the  fruit  grows 
older  and  at  last  nearly  disappear,  though  traces  of  them  are  seen  in  the 
pentagonal  or  hexagonal  form,  in  cross-section,  of  older  fruits  (figs.  3, 
26,  43). 

The  internal  structure  of  the  fruit  is  far  more  variable  than  that  of  the 
exterior.  It  may  have  no  seeds  at  all  or  it  may  contain  anywhere  from  1  ta 
200  or  more  seeds.  These  seeds  may  all  be  shriveled,  partially  developed 
rudiments,  or  from  a  few  to  most  of  them  may  possess  normally  matured 
embryos.  The  number,  condition,  and  structure  of  the  seeds  seem  to  show 
no  correlation  with  the  external  form  of  the  fruit  (figs.  25,  26,  28). 

The  most  striking  peculiarities  of  the  fruit  of  this  Opuntia  are,  as 
suggested  above,  its  failure  to  ripen,  its  persistence  on  the  plant,  its  long- 
continued  gi'owth,  and  finally  its  capacity  for  proliferation,  whether  left 
attached  to  the  parent  plant  or  torn  loose  from  it. 

THE  FRUIT  AT  THE  TIME  OF  ABSCISSION  OF  THE  PERIANTH. 

With  the  dropping  from  the  ovary  of  the  stamens  and  perianth  we  have 
left  the  earliest  stage  of  the  fruit  proper.  Externally,  the  fruit  so  formed  is 
a  globular  or  obconical  structure,  with  a  very  deep,  funnel-like  depression  in 
the  top  and  from  15  to  40  strongly  raised  tubercles  on  its  sides.  Each  of  the 
latter  bears  a  leaf-scar  and  a  more  or  less  developed  areole  at  its  upper  end, 
which  was  mentioned  in  describing  the  ovary  of  the  flowers. 

In  internal  structure  this  young  fruit  consists  of  the  superficial  epidermis, 
which  is  soon  continued  over  the  leaf-scars  and  perianth-scars  by  a  corky 
periderm.  Below  the  epidermis  is  the  4  or  5  layered  hypodennis,  consisting 
of  an  outer  layer  of  crystal-holding  cells,  and  within  this  of  3  or  4  layers  of 
collencliymatously  thickened  cells,  in  which  the  cell-cavity  is  finally  to  be 
nearly  obliterated  (figs.  71,  72).     Within  the  hypodermis  are  the  elabor- 


26  THE   FEUIT   OF   OPUNTIA   FULGIDA. 

ately  ventilated,  photosyntlietic  palisade  of  the  cortex,  with  its  scattered 
slime-cells  and  crystal  cells,  and  the  complexly  reticulated  vascular  system ; 
and,  finally,  within  the  latter  and  between  its  meshes  are  many  layers  of 
mucilaginous  storage  parenchyma.  The  latter  in  turn  surrounds  the  cavity 
of  the  ovary,  now  nearly  filled  by  the  young  seeds  and  by  the  mass  of  loose 
tissue  arising  from  their  long,  coiled,  funicular  strands  (fig's.  23_,  40,  71). 

The  somewhat  uneven  outer  layer  of  the  epidermis  of  the  projecting 
tubercles  consists  at  this  stage  of  rather  cuboidal  cells,  with  bulging  and 
considerably  thickened  outer  walls  (fig.  71).  The  epidermal  cells  of  the 
gTooves  between  the  tubercles  are  somewhat  elongated  radially  and  thinner- 
walled  than  those  of  the  outer  margins  of  the  tubercles  just  described. 
Stomata  are  scattered  rather  frequently  over  the  surface  of  the  tubercles. 
The  guard-cells  are  already  sunken  considerably  below  the  surface,  though 
not  as  far  as  in  the  mature  fruit  (figs.  71,  72).  The  inner  layer  of  the  epi- 
dermis already  includes  many  crystal-containing  cells  of  the  sort  figured  in 
detail  in  the  older  fruit  (fig.  71,  72,  73). 

The  cork  of  the  fruit  at  this  stage  is  confined  to  the  pit-like  scar  at  the 
top  of  the  fruit,  where  it  forms  an  ashy-white  layer.  It  consists,  outside  the 
phellogen,  of  above  8  to  10  layers  of  rectang-ular  cells  approximately  100 
microns  long  and  broad  and  20  to  40  microns  thick,  radially.  The  walls  of 
all  save  one  layer  of  these  are  not  greatly  thickened,  nor  apparently  strongly 
suberized  (fig.  63).  The  very  bottom  of  this  cup  around  the  stump  of  the 
style  is  commonly  still  protected  only  by  the  original  epidermis,  which,  as 
was  noted  earlier,  is  often  not  cast  off  along  with  that  part  of  the  lining  of  the 
cup  which  bears  the  stamens  (figs.  23,  60). 

The  photosynthetic  tissue  of  the  ovary-wall  has  already  beg-un  to  assume 
the  striking  arrangement  in  radial  rows  that  is  so  characteristic  of  the 
cortical  tissue  of  the  mature  fruit  as  well  as  of  the  vegetative  joint  (figs.  39, 
70,  71,  73).  Scattered  abundantly  through  this  palisade,  which  extends 
inward  8  or  10  cells  from  the  surface,  are  numerous  mucilage-cells  of  the 
usual  more  rounded  form. 

The  vascular-bundle  system  of  the  young  fruit  consists  of  about  16  pri- 
mary bundles  entering  it  at  the  base.  These  soon  divide  by  dichotomous 
forking  to  form  twice  as  many  biuidles  at  the  level  of  the  lower  end  of  the 
ovarian  cavity.  The  repetition  of  this  forking  gives  rise  in  the  upper  half 
of  the  fruit  to  a  still  more  complex  system  of  main  vascular  bundles  (figs. 
17,  21,  25,26).  From  the  points  of  forking  of  these  main  bundles  are  given 
off  the  groups  of  smaller  bundles,  one  group  at  the  base  of  each  tubercle,  to 
supply  the  tubercle,  its  growing-point,  and  the  leaf  and  nectaries  arising 
from  it  (figs.  21,  27,  28,  32).  The  main  vascular  bundles  are  nearly  isodia- 
metric  in  cross-section  at  this  stage  of  development,  and  are  about  500 
microns  thick  radially  and  300  microns  broad  tangentially  (fig.  44a).  In 
internal  structure,  which  may  be  noted  briefly  for  comparison  with  the 
bundle  of  the  older  fruit,  each  main  bundle  includes  about  25  or  30  tangen- 


THE  FRUIT   OF   OPUNTIA  FULGIDA.  27 

tial  layers  of  xjlem  elements.  Outside  of  these  lies  a  group  of  6  or  8  layers 
of  radially  arranged  cambium-cells  and  their  little  modified  derivatives. 
Still  further  outward  on  the  same  radius  is  the  half-cylindrical  strand  of 
phloem-cells,  about  15  to  20  cells  thick  radially  (fig.  44a).  From  a  ring 
of  anastomosed  bundles  near  the  rim  of  the  funnel  at  the  top  of  the  fruit 
a  series  of  downward-growing  branches  gives  rise  to  the  6  or  7  pairs  of 
placental  bundles,  one  pair  running  longitudinally  behind  each  double  row 
of  ovules  (figs.  11,  22,  23,  26,  28,  38,  39,  41).  From  this  same  vascular 
ring  numerous  small  branches  grow  inward  and  upward.  These  are  the 
ones  that  supplied  the  stamens  and  perianth  members  before  they  were  shed. 
Other  similar  branches  grow  still  farther  downward  and  then  turn  upward 
to  form  the  6  or  7  bundles  of  the  style  and  stiginas  (figs.  20,  22,  23,  28,  37, 
41,61). 

The  most  important  structures  within  the  fruit — the  seeds — arise,  as  we 
have  seen,  in  a  double  row  on  each  of  the  6  or  7  placentae.  There  are  from  8 
to  12  or  more  young  seeds  in  each  row,  making  from  100  to  sometimes  200 
or  more  seeds  that  are  initiated  in  the  finiit  as  a  whole.  Usually  not  all  of 
these  mature.  Sometimes,  as  noted  above,  none  of  them  mature.  At  the 
time  the  perianth  is  dropped  there  is  a  decided  difference  in  size  among  the 
seeds  present  in  any  one  ovary.  The  most  developed  are  about  0.5  mm.  in 
diameter  inside  the  pocket  of  the  funiculus  in  which  each  seed  is  inclosed 
and  about  0.25  mm.  thick.     The  seed-coats  are  unthickened  and  have  only 

2  layers  of  cells  each.  The  embryo  sac  has  usually  reached  the  8-nucleate 
stage.  The  cavity  of  the  ovary  is  at  this  time  about  half  filled  with  the  seeds 
and  the  swollen  funiculi  or  seed-stalks. 

THE  MATURE  FRUIT. 

At  the  end  of  the  growing-season,  which  in  Tucson  terminates  about  the 
middle  of  October,  the  fruit  has  attained  what  must  be  regarded  as  a  mature 
stage.    The  seeds  within  it  are  now  full  grown.    These  mature  fruits,  of  from 

3  to  5  months'  growth,  differ  considerably  in  shape  and  size.  They  are 
commonly  pear-shaped,  or  somewhat  more  rounded,  and  have  a  diameter  of 
12  to  15  mm.,  with  a  length  of  20  to  35  mm.  or  more  (figs.  3',  7,  8).  Later 
fruits  of  the  season  may  be  as  large  as,  or  even  larger  than,  the  earlier  ones. 
Fruits  of  one  season  may  average  in  all  dimensions  larger  than  those  of  the 
season  before. 

The  surface  of  the  typical  fruit  at  maturity  has  filled  out  so  that  the 
tubercles  have  lost  their  prominence.  The  areoles  differ  in  size  on  the  same 
fruit,  the  upper  or  larger  ones  at  this  time  being  15  to  30  or  even  35  mm.  in 
diameter.  Occasionally  an  elongated  fruit  will  be  formed  which  has  much 
the  appearance  of  a  mammillate  vegetative  joint,  except  for  the  presence  of 
the  perianth-scar  at  the  top  and  the  fact  that  the  spines  are  weaker  than  in 
most  joints  (fig.  7). 

The  most  important  features  of  the  internal  structure  of  the  fruit  are  those 
concerned  with  the  vascular  bundles  of  the  wall  of  the  ovary  and  with  the 


28  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

character  and  contents  of  the  ovarian  cavity.  The  main  vascular  bundles  of 
the  wall  increase  only  moderately  after  the  fall  of  the  perianth,  though  they 
do  increase  somewhat  in  radial  dimensions  by  the  addition  of  new  layers  of 
xylem  tissues  (fig.  44&).  The  amount  of  the  cortical  parenchyma  outside 
the  vascular  bundles  may  also  increase  somewhat,  and  this  latter  growth  is 
probably  the  cause  of  the  rounding  out  of  the  fruit  and  the  disappearance  of 
the  mammillse  (figs.  3,  76,  8). 

The  number  and  character  of  the  seeds  present  in  different  fruits  differ 
markedly.  There  may  sometimes  be  an  ovarian  cavity  filling  three-fifths  of 
the  diameter  of  the  fruit,  or  this  cavity  may  be  practically  wanting.  About 
half  the  mature  fruits  contain  one  or  more  ripe  seeds,  which  vary  in  number 
from  1  to  100  or  200  per  fruit.  The  other  half  of  the  fruits  have  only  small 
ovarian  cavities  in  which  the  seed  rudiments  have  ceased  their  development 
at  different  stages  from  half-formed  ovules  up  to  half -grown  seeds  (fig.  23). 
In  some  cases  not  even  these  withered  rudiments  can  be  found  in  the  place 
where  the  ovarian  cavity  should  be. 

A  consideration  of  the  facts  related  inclines  at  first  to  the  conclusion  that 
ovaries  mature  into  fruits  only  when  pollination  has  occurred  and  that  seeds 
form  only  when  the  further  process  of  fertilization  has  taken  place.  This 
seems  doubtful,  however,  in  view  of  the  fact  that  many  seeds  may  go  half- 
way through  their  development  before  degeneration  sets  in.  The  attempt  is 
being  made  to  determine  the  distribution  of  these  different  types  of  degenera- 
tion in  different  plants  and  different  flowers  of  the  same  plant. 

The  most  striking  peculiarity  of  these  mature  fruits  is  that  they  do  not 
ripen.  From  analogy  with  all  other  fruits,  we  should  expect  at  this  time 
that  the  flesh  of  the  Opuntia  fruit  would  either  change  color  and  soften  or 
harden  up  to  form  a  dry  fruit.  But  nothing  of  this  sort  happens.  The 
matured  fruit,  with  its  color  still  bright  green,  with  its  photosynthetic  tissues 
still  active  in  starch-making,  and  its  well-established  fascicular  cambium 
simply  halted  until  the  next  spring,  enters  into  the  resting-period  of  4  or  5 
months.  There  is  not  the  slightest  sign  of  ripening  or  any  change  at  all 
comparable  with  this  process.  There  is  no  preparation  of  any  sort  for  the 
discharge  or  escape  of  the  seeds  to  conditions  conducive  to  germination. 
On  the  contrary,  the  majority  of  the  mature  fruits  normally  remain  attached 
to  the  parent  shoot,  not  merely  through  the  succeeding  fall  and  winter,  but 
season  after  season  for  several  or  many  years.  These  persistent  fruits 
become  essentially  a  part  of  the  vegetative  shoot,  performing  not  only  the 
photosynthetic  function  of  a  vegetative  joint,  but  also  budding  out  new 
shoots.  Usually  these  shoots  arising  from  attached  fruits  are  floral  shoots. 
More  rarely  a  vegetative  joint  may  arise  from  such  a  fruit,  and  then  the 
latter  becomes  a  constitutent  joint  of  the  vegetative  branch,  undistingiTish- 
able  except  by  its  perianth-scar  and  usually  by  its  lack  of  spines  (fig.  79). 
The  fate  of  a  fruit  in  these  respects  is  the  same,  whether  it  contains  many 
fertile  seeds  or  whether,  as  may  often  be  the  case,  it  is  entirely  seedless. 


THE   FRUIT   OF   OPUNTTA   FULGIDA.  29 

THE  PERENNATING  FRUIT:    ITS  STRUCTURE  AND  SECONDARY  GROWTH. 

The  matured  fruits  of  Opuntia  fulgida  not  only  do  not  ripen  at  the  end  of 
the  growing-season,  but  (as  noted  above)  they  are  neither  shed  from  the 
plants  as  most  fruits  are,  nor  do  they  open  in  any  way  to  discharge  their 
seeds.  On  the  contrary,  these  fruits  remain  year  after  year,  attached  and 
actively  growing,  until  they  may  become  40  or  50  mm.  in  diameter  and  70 
or  80  mm.  long.  The  growth  of  these  fruits  in  length  must  be  chiefly 
primary ;  that  is,  each  fruit  attains  practically  its  maximum  length  during 
its  first  season's  growth.  Thus  the  longer  fruits  mentioned  must  have  been 
exceptionally  long  at  the  start. 

Growth  in  thickness  of  the  larger  fruits  is,  on  the  contrary,  largely 
secondary.  It  is  accomplished  by  the  persistent  activity  of  the  fascicular 
cambium  and  by  the  general  multiplication  of  the  parenchyma  cells  between 
the  bundles.  Whether  there  is  an  increase  in  the  central  medullary 
parenchyma  of  the  fruit  has  not  been  determined  with  certainty,  but 
there  appears  to  be  some  little  increase  in  the  interior  diameter  of  the 
vascular  ring.  The  cortical  parenchyma,  inside  the  hypoderm,  may  in- 
crease in  thickness  from  about  2  mm.  in  the  just-matured  fruit  to  5  mm. 
or  more  in  a  fruit  several  years  of  age.  This  radial  growth  seems  to  be 
almost  entirely  a  result  of  the  radial  elongation  of  the  cortical  cells,  as  the 
number  of  these  along  a  radius  ranges  from  20  to  25  in  both  the  just-matured 
fruit  and  in  that  3  or  4  years  old. 

The  fascicular  cambium  of  the  perennating  fruit  gives  rise  to  numerous 
phloem  and  xylem  elements,  which  increase  the  radial  dimension  of  the 
bundle  from  O.Y  or  0.8  mm.  in  the  newly  matured  fruit  to  3  or  3.5  mm.  in  a 
fruit  several  years  old.  The  radial  extent  of  the  xylem  in  the  newly  formed 
fruit  is  about  0.6  mm.  and  that  of  the  phloem  0.2  mm.,  while  in  an  older 
fruit  the  dimensions  are  2.6  mm.  for  the  xylem  and  0.6  mm.  for  the  phloem. 
This  shows  that  the  growth  of  the  two  tissues  has  kept  their  bulk  much  the 
same  relatively.  The  tangential  growth  of  the  bundles  is  relatively  slight, 
the  outer  edge  of  a  bundle  4  or  5  years  old  being  only  half  as  wide  again  as 
that  of  one  in  the  first-year  fruit.  ISTo  definite  annual  rings  could  be  dis- 
tinguished in  the  wood  of  these  older  fruits.  The  number  of  larger  xylem 
elements  along  a  single  radius  in  a  young  fruit  is  about  25,  while  in  a  4  or  5 
year  fruit  there  may  be  80  or  90  vessels  on  a  single  radius. 

The  parenchyma  of  the  pith-rays  differs  from  that  of  the  cortex  by  show- 
ing a  vigorous  cell  multiplication.  The  pith-ray  of  the  just-matured  fruit 
may  have  but  8  or  10  cells  in  the  radial  extent  of  the  bundle,  while  in  a 
fruit  4  or  5  years  old  a  pith-ray  may  be  made  up  of  25  or  30  cells  in  the 
radial  width  of  the  bundle.  This  growth  is  apparently  due  to  the  general 
division  of  cells  throughout  the  ray,  as  no  trace  of  an  interfascicular 
cambium  is  discoverable. 

Two  structures  only  on  the  surface  of  the  fruit  undergo  important  changes 
as  the  fruit  ages.     These  are  the  corky  periderm  and  the  areoles.     At  the 


30  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

end  of  the  season  of  its  origin  the  fruit  is  protected  by  the  well-cutinized 
epidermis  and  the  underlying  hypodermis,  except  for  the  perianth-scar  and 
the  individual  leaf-scars.  This  epidermis  persists  over  the  general  surface 
of  the  fruit  for  several  years.  Only  where  the  epidermis  is  cracked  by  the 
swelling  of  the  fiTiit  or  is  otherwise  injured  is  the  original  protective  layer 
replaced  in  function  by  cork,  like  that  of  the  perianth-scar. 

The  periderm  of  the  perianth-scar  has  its  origin,  as  we  have  noted,  from 
a  pliellogen  layer  arising  in  descendants  of  the  original  cells  of  the  abscission 
layer,  just  within  the  plane  of  abscission  (figs,  62,  63).  The  first  5  or  6 
layers  of  cork  cells  formed  by  the  cork  cambium  are  very  thin-walled ;  then 
there  is  formed  a  single  layer  of  very  thick  cells  (see  Wolfe,  1912,  figs.  6, 
7,  9).  These  sclerenchyma-like  cells  have  walls  of  a  pale-yellow  color, 
which  in  older  fruits  are  made  up  of  from  15  to  20  distinct  layers  marked  by 
numerous  minute  radial  pits  (figs.  63,  64,  65).  The  walls  stain  intensely 
with  safranin  and  gentian  violet.  The  cork  cambium,  after  forming  this 
thick  layer,  may  continue  to  form  more  thin-walled  cork  cells  until,  in  the 
older  fruits,  12  to  15  or  more  layers  are  present  inside  the  intact,  thick  layer. 
These  later  layers  are  more  numerous  on  the  upper  margin  of  the  perianth 
scar.  With  increasing  age  the  5  or  6  layers  of  thin  cork  outside  the  scleren- 
chyma  layer,  which  are  left  more  or  less  shriveled  after  abscission,  are  grad- 
ually worn  off  from  the  more  exposed  parts  of  the  scar.  This  leaves  the 
sclerenchyma  as  the  superficial  and  essentially  protective  layer  unless  this  is 
injured.  When  this  primary  sclerenchyma  layer  is  broken  a  secondary  one 
is  formed  immediately  outside  the  then  active  phellogen  (fig.  14,  at  right). 

Periderm  formation  in  the  region  of  the  areole  begins  soon  after  the  fall 
of  the  leaf,  in  an  area  that  includes  tissue  of  the  leaf -scar  itself  and  a  few 
of  the  epidermal  cells  below  and  beside  this  (fig.  14).  There  is  thus  formed 
a  periderm  of  from  3  or  4  to  8  or  10  cells  in  thickness,  of  which  the  portion 
immediately  above  the  foliar  bundle  may  finally  include  3  or  even  4  immedi- 
ately superposed  sclerenchyma  layers  (figs.  63,  64).  As  development  of  the 
areole  proceeds  the  cork  layer  develops  upward  from  the  leaf-scar  and 
thin-walled  cork-like  but  apparently  little-suberized  cells  are  formed  imme- 
diately beneath  the  persistent  trichomes,  just  above  the  leaf.  Later  the 
formation  of  these  cells  continues  on  beneath  the  bases  of  the  withered  nec- 
taries and  spines  if  there  are  any,  and  finally  beneath  the  bristles  (figs.  14, 
17).  The  latter,  in  older  fruits,  may  be  supported  upon  12  to  15  or  more 
layers  of  clear  thin-walled  cork-like  cells  (fig.  14).  The  only  parts  of  the 
areoles  that  are  not  covered  are  the  growing-point  and  the  immediately  sur- 
rounding series  of  rudiments  of  nectaries,  bristles,  etc.  These  all  lie  well- 
protected  at  the  base  of  the  dense  tuft  of  younger  tricbomes  that  overhang 
the  growing-point  (fig.  50). 

On  the  lateral  surface  of  the  older  fruit  corky  tissue  may  arise  by  the 
extension  of  the  cork  about  the  areole,  or  that  of  the  perianth-scar  over  the 
edge  of  the  cup,'  or  it  may  arise  de  novo  from  injured  spots  in  the  clear  epi- 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  31 

dermis  between  the  areoles.  In  this  way  considerable  portions  of  the  sur- 
face of  a  fruit  4  or  5  years  old  may  become  covered  with  cork  developed 
below  cracks  or  bruised  spots,  and  the  fruit  thus  come  to  have  the  mottled 
green  and  gray  color  characteristic  of  maturing  vegetative  joints  (fig.  3). 
The  effect  of  this  cork,  aside  from  affording  protection  to  the  water-stored 
cortical  tissues,  must  be  also  to  cut  off  the  light  and  air  from  the  photosyn- 
thetic  cells  of  the  portion  of  the  fruit  covered  by  it.  While  lessening  its 
starch-making  capacity,  the  cork  is  thus  of  prime  importance  to  the  fruit  in 
maintaining  it  as  a  supporting  and  conducting  structure  for  the  flowers  and 
persistent  fruits  that  may  arise  from  it  year  after  year. 

The  structure  of  the  areole  in  the  young  fruit  has  already  been  discussed 
(p.  12).  The  chief  change  in  the  areole  with  increased  age  of  the  fruit 
is  the  increase  in  size,  until  it  may  become  3  or  4  mm.  broad  and  6  or  8  mm. 
long  in  the  case  of  those  near  the  top  of  the  fruit.  This  increase  in  size  is 
due  chiefly  to  the  continued  production  of  hundreds  of  trichomes  year  after 
year.  These  appear  in  rather  definite  concentric  circles  around  the  growing- 
point.  Besides  the  trichomes,  the  areole  also  forms  additional  nectaries, 
year  after  year,  till  at  least  a  score  may  successively  develop  and  wither. 
The  bristles  of  the  fertile  areole,  as  has  been  noted,  are  not  increased  in  num- 
ber after  a  flower  is  initiated  (figs.  14,  17,  50).  The  sort  of  development 
here  indicated  is  the  characteristic  one  for  many  of  the  areoles  of  the  upper 
half  of  the  ovary  throughout  the  life  of  the  fruit.  Others  of  the  basal  half 
or  quarter  of  the  fruit  may  persist,  but  show  only  slight  growth  for  2  or  3 
years  and  then  cease  growing  altogether. 

Other  of  the  upper  areoles,  several  in  each  fruit,  give  rise  to  one  or  the 
other  of  the  two  most  important  structures  developed  from  fruits.  These 
are  the  floral  shoots  that  may  arise  on  the  attached  fruit  and  the  vegetative 
shoot  that  develops  from  the  areole  of  a  detached  fruit.  These  structures 
are,  of  course,  the  normal  products  of  axillary  buds.  Hence  the  earlier 
period,  during  which  these  areoles  are  developing  only  modified  structures 
like  bristles  and  nectaries,  must  be  regarded  as  a  sort  of  resting-period  in 
which  the  normal  activity  of  the  bud  is  inhibited. 

When  the  normal  development  of  an  areole  is  accomplished  and  a  shoot 
produced,  all  further  activity  by  this  areole  is  terminated,  as  must  be  evident 
from  the  fact  that  the  whole  mass  of  the  single  growing-point  of  the  areole  is 
embodied  in  either  the  vegetative  shoot  or  the  flower.  The  exact  conditions 
conducive  to  the  production  of  a  flower  in  one  case  and  a  vegetative  shoot  in 
another,  with  the  details  of  development  of  each,  will  be  described,  as  far 
as  kno^\Tl,  in  the  discussion  of  proliferation  (see  pp.  35-50). 

The  possible  activities  of  the  fruit  of  Opuntia  fulgida  may  be  summarized 
as  follows:  The  fruit  usually  remains  attached  and  in  succeeding  seasons 
buds  out  new  flowers.  If  fruits  fall  to  the  ground  from  increasing  weight 
of  the  cluster  or  when  dislodged  by  wind  or  brow^sing  animals,  then  any  one 
of  three  things  may  happen.    Most  frequently  the  fruit  dries  up  or  decays 


32  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

and  nothing  comes  of  it.  If  it  drops  on  moderately  moist  soil,  then  it  may 
give  rise  to  roots  and  shoots  bv  proliferation  from  the  areoles.  More  rarely 
still,  the  fruit  may  set  the  seeds  free  by  decay  and  the  latter  may  possibly 
then  germinate  to  seedling  plants.  This  latter  fate,  which  must  be  regarded 
as  the  most  normal  one  for  such  a  fruit,  is  evidently  a  very  rare  one  for  the 
fruit  of  Opuntia  fulgida,  at  least  in  the  deserts  near  Tucson,  v^^here  seedlings 
have  never  been  seen. 

THE  SEED:   ITS  STRUCTURE,  PERSISTENCE.  AND 
GERMINATION. 

The  ripe  seed  is  the  one  essential  structure  of  the  fruit  which  remains 
entirely  unchanged  year  after  year,  as  the  fruit  persists  on  the  parent  plant. 

The  mature  or  ripe  seed  is  an  irregular,  round-angled,  flattened  disk  about 
5  mm.  in  diameter  and  il.5  to  2  mm.  thick  in  the  middle  (fig.  77).  It  usually 
bulges  on  both  sides  of  the  disk,  but  often  far  more  on  one  side  than  the 
other.  The  majority  of  good  seeds  are  pale  yellow  in  color,  with  remnants 
of  many  colorless  cells  from  the  fleshy  ovule-stalks  sticking  to  them.  In 
internal  structure  the  seed  consists  of  a  well-developed  curved  embryo  bent 
in  the  plane  of  the  disk-like  seed,  of  a  small  remnant  of  endosperm  near  the 
center  of  the  seed  and  embraced  by  the  bent  embryo,  and  finally  of  a  protect- 
ing j  acket,  formed  chiefly  by  the  layers  of  tissue  arising  from  the  pocket  of 
the  funiculus,  but  including  also  the  two  thin  integuments  (figs.  76,  77). 

The  embryo  is  from  0.7  to  1  mm.  in  diameter  and  about  4  or  5  mm.  long. 
It  is  bent  in  the  plane  perpendicular  to  that  of  the  adjacent  faces  of  the  coty- 
ledons. Most  of  the  cells  of  both  cotyledons  and  radicle  of  the  embryo  are 
densely  stored  with  what  appear  to  be  aleurone  grains  and  slime  globules. 
These  bodies  take  on  a  brownish  and  not  a  bluish  color  with  iodine,  and 
show  structural  features  not  characteristic  of  starch-grains.  Occasional 
cells  of  the  cotyledons  and  upper  part  of  the  radicle  are  completely  occupied 
each  with  a  large  cr^^stal  of  calcium  oxylate  like  those  found  in  the  paren- 
chyma of  the  mature  plant. 

The  endosperm,  which  consists  of  a  small  mass  in  the  bend  between  radicle 
and  cotyledons  and  of  thinner  layers  extending  along  beside  the  radicle  and 
over  the  tips  of  the  cotyledons  (fig.  98),  is  densely  stored  with  starch  which 
reacts  in  the  usual  way  with  iodine. 

Small  fragments  of  perisperai,  left  in  the  corners  where  the  embryo  fits 
the  integument  less  accurately,  are  filled  with  granules  reacting  like  those  in 
the  embryo  itself.  The  cell-contents  of  both  embryo  and  endosperm  seem  to 
remain  entirely  unchanged  in  fruits  that  have  persisted  on  the  plant  for 
many  years  after  the  maturing  of  the  seeds.  The  integuments  are  made  up 
of  considerably  thickened  cells,  the  outermost  layer  of  them  with  w^avy 
brown  walls. 

The  jacket  derived  from  the  funiculus  surrounds  the  seed  completely, 
having  finally  closed  in  above  the  micropyle  (fig.  97).     It  makes  up  more 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  33 

than  nine-tentlis  of  the  thickness  of  the  coat  surrounding  the  seed  and  con- 
sists of  three  distinct  layers.  The  inner  layer  is  from  150  to  300  microns 
thick.  It  consists  of  intertwisted  threads  or  fibers  which  run  meridionally 
about  the  seed.  The  component  cells  of  these  threads  are  8  to  10  microns  in 
diameter  each  by  150  or  200  microns  long.  The  middle  layer  of  the  funi- 
cular pocket  is  made  up  of  somewhat  similar  interwoven  threads  running 
equatorially,  i.  e.,  around  the  luargin  of  the  disk-like  seed.  The  cells  of  this 
layer  are  20  to  25  microns  in  diameter,  but  only  50  to  60  microns  long.  The 
third  or  outermost  layer  of  the  seed-coat  is  very  uneven,  consisting  of  a  single 
layer  of  cuboidal  cells  30  or  40  microns  thick,  about  most  of  the  margin  of 
the  seed,  but  of  Y  or  8  layers  with  a  total  thickness  of  400  microns  on  the 
sides  of  the  seed.  It  is  this  greater  thiclmess  of  the  outer  funicular  layer 
that  makes  up  most  of  the  bulge  on  the  flat  side  of  the  disk-like  seed. 

The  whole  structure  and  condition  of  the  seed  apparently  remain  quite 
unchanged  year  after  year  so  long  as  the  fruit  containing  it  remains  attached 
to  the  tree.  Not  only  are  the  embryos  of  seeds  from  the  oldest  attached 
fruits  still  plump,  with  the  stored  starch  intact,  but  a  seed  from  such  an  old 
f niit  is  just  as  capable  of  germinating  as  one  from  a  fruit  just  matured. 

A  very  important  question  arising  in  this  connection  is :  Why  do  not 
these  seeds,  immersed  in  the  moist  pulp  of  the  parent  fruit  and  raised  each 
summer  to  a  relatively  high  temperature,  germinate  there,  without  waiting 
to  escape  from  the  fruit  ?  The  attempt  was  made  to  germinate  seeds  under 
these  conditions  by  treating  them  in  the  manner  which  was  found  necessary 
to  secure  germination  outside  the  fruit,  that  is,  by  chipping  the  seed-coat. 
Seeds  with  the  coats  cut  off  at  one  edge  Avere  carefully  inserted  in  the  pulp 
of  sound  fruits  witli  every  precaution  not  to  injure  the  fruit  more  than  neces- 
sary. The  wound  was  then  sealed  with  vaseline  and  set  in  a  moderately 
moist  chamber  to  induce  germination.  No  germinations  of  seeds  under 
these  conditions  were  secured.  In  each  case,  whether  the  injured  fruit 
dried  out,  decayed,  or  took  root  in  the  soil,  the  embrj^o  of  the  inserted  seed 
after  some  weeks  blackened  and  died.  The  explanation  of  this  failure  of 
the  seed  to  germinate  in  its  oavh  fruit  or  in  the  host  fruit  was  not  discovered. 
It  is,  of  course,  possible  that  it  may  be  found  in  the  mere  exclusion  of  oxygen 
by  the  seed-coat  or  by  the  mucilaginous  pulp  of  its  OA\m  or  the  host  fruit. 
On  the  other  hand,  it  is  quite  possible  that  the  osmotic  or  chemical  character 
of  the  cell-sap  of  the  pulp  surrounding  the  seeds  is  the  real  obstacle  to 
germination. 

During  two  sojourns  at  Tucson  the  attempt  was  made  to  test  the  common 
report  that  seedlings  of  Opuntia  fulgida  do  not  occur  in  the  field  about 
Tucson  (Tourney,  1905,  p.  360).  Diligent  search  was  made  for  them  in 
many  groves  of  these  trees  in  the  months  of  April  and  May  of  1912  and 
1915,  but  not  a  single  imdoubted  seedling  of  this  species  was  discovered. 
Examination  of  large  numbers  of  fallen  fniits  showed  no  sign  of  germina- 
tion of  the  seeds.  It  then  occurred  to  the  writer  that  a  search  on  a  cattle 
3 


34  THE   FRUIT   OF   OPUNTIA  FULGIDA. 

range  outside  the  protected,  ungrazed  property  of  the  Desert  Laboratory 
might  show  that  seedlings  arose  from  seeds  that  had  passed  through  the  ali- 
mentary canals  of  cattle  that  had  eaten  the  fruits.  This  search  also  proved 
futile.  It  is  of  course  possible  that  seedlings  may  occur  in  other  parts  of 
the  area  of  distribution  of  this  opuntia,  or  in  the  Tucson  region  itself,  at 
other  seasons. 

With  the  hope  of  discovering  something  regarding  the  cause  or  causes  of 
this  failure  of  the  seeds  to  germinate  naturally,  the  attempt  was  made  to 
germinate  them  artificially.  In  the  first  place,  dozens  of  plump  seeds  likely 
to  possess  good  embryos  were  repeatedly,  in  spring,  autumn,  and  winter, 
sown  on  soil  or  on  filter-paper,  and  put  either  in  a  warm  gi'eenhouse  or  on  a 
warm  bath  at  30°  to  35°.  'Not  a  single  germination  was  obtained  from 
these  experiments. 

Other  series  of  somngs  were  made  of  plump  and  apparently  fertile  seeds 
from  which  the  seed-coat  had  been  cut  or  filed  away  at  one  part  of  the 
margin.  Out  of  several  dozens  of  these  cut  seeds  so^m  between  layers  of 
damp  filter-paper  in  a  gallon  battery-jar,  only  about  8  or  10  germinated. 
The  best  proportion  of  germinations  obtained  was  5  embryos  out  of  25  prob- 
ably fertile  seeds. 

The  difficulties  in  determining  what  proportion  of  seeds  are  capable  of 
germination  are  two.  In  the  first  place,  it  is  impossible  to  tell  from  its 
external  appearance  whether  a  seed  has  a  normal,  well-developed  embryo  or 
not.  Quite  aside  from  the  half -developed  and  withered  seeds,  there  are 
many  of  mature  size,  and  apparently  plump  and  healthy,  which  upon  being 
cut  open  reveal  no  embryo  at  all,  or  a  discolored  and  shriveled  one  with  no 
stored  material  in  it.  Secondly,  of  the  seeds  that  are  cut  into  far  enough 
to  determine  whether  they  contain  good  embryos,  or  far  enough  to  make 
certain  the  escape  of  the  swelling  embryo  from  within,  some  may  be  so 
injured  as  to  prevent  germination  of  any  sort. 

The  trials  made  thus  far  show  that  a  certain  relatively  small  percentage  of 
the  seeds  of  this  opuntia  are  fertile  and  capable  (if  aided  in  getting  out  of  the 
coat)  of  producing  normal  seedlings  (fig.  Y6). 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  35 

PROLIFERATION  OF  FLOWERS  AND  FRUITS. 

The  most  unique  peculiarity  of  the  flower  or  fruit  of  Opuntia  fulgida  is 
its  ability  to  produce  new  shoots.  These  shoots  may  be  floral  shoots  only, 
as  in  the  case  of  the  attached  flower  or  fruit,  or  they  may  be  vegetative  shoots, 
as  in  the  case  of  detached  fruit.  This  capacity  for  proliferation  is  depend- 
ent, as  has  been  noted,  on  the  presence  and  persistence  of  the  axillary  buds  or 
areoles  of  the  wall  of  the  ovary.  These  axillary  buds  are  unknown,  as  far  as 
the  writer  has  been  able  to  leam,  on  the  ovary  of  any  other  family  of  plants, 
and  even  in  the  Cactacese  occur  only  in  the  genera  Opuntia,  Nopalea,  and 
Peireskia. 

The  structure  of  these  areoles,  with  a  description  of  the  rudiments  present 
in  them,  has  already  been  noted  (p.  11).  We  have  seen  also  that  most 
of  the  areoles  of  a  flow^er  and  fruit  may  persist  year  after  year  without 
developing  anything  but  minor  organs,- such  as  nectaries,  trichomes,  and 
spicules.  We  have  now  to  consider  the  very  important  function  perfoiined 
by  certain  of  the  areoles  in  their  proliferation  to  shoots  of  limited  or 
unlimited  growth.  There  are  three  types  of  proliferation  by  the  areoles  of 
the  ovary  of  this  opuntia.  In  the  first  place,  from  1  to  5  or  more  of  the 
areoles  of  the  unopened  flower  may  give  rise  to  the  buds  of  secondary  flowers, 
which  open  soon  after  the  primary  ones.  Secondly,  one  or  several  areoles  of 
an  attached  fruit  may  in  the  first  or  in  some  later  season  after  its  develop- 
ment give  rise  to  primary  flowers  of  that  season.  Thirdly,  the  areoles  of  a 
detached  fruit  that  has  been  separated  from  the  parent  plant  after  maturing 
and  placed  on  moist  soil  may  give  rise,  first  to  adventitious  roots  and  then 
later  to  the  characteristic  joints  of  vegetative  shoots.  The  fruit  thus  per- 
forms the  unique  function  of  initiating  a  new  plant  by  purely  vegetative 
propagation. 

PROLIFERATION  FROM  ATTACHED  FLOWER  BUDS. 

The  primary  flowers  of  the  season  arise  in  either  April  or  May  on  either 
the  vegetative  joints  or  the  fruits  of  the  preceding  season  or  sometimes  on 
those  of  a  still  earlier  season.  These  flowers  are  developed  from  the  growing- 
points  of  the  upper  areoles  of  the  joint  or  fniit  (fig.  47)  in  the  manner  noted 
above  (p.  9). 

Before  the  primary  flower  is  half-grown  the  buds  of  secondary  flowers  of 
various  sizes  can  be  detected  developing  in  from  1  to  4  or  5  of  its  upper 
areoles  (figs,  ^a,  9&,  13).  By  the  time  the  primai-y  flower  is  ready  to  open, 
soon  after  the  middle  of  May  at  Tucson,  the  larger  buds  of  the  secondary 
flowers  are  about  one-sixth  grown,  and  are  recognizable  as  buds  of  flowers 
rather  than  of  vegetative  shoots  (figs.  9a,  9&,  47).  About  4  weeks  after  the 
primary  flower  withers  and  sheds  its  perianth,  the  largest  secondary  flower  in 
its  turn  opens.  This  occurs  usually  between  the  middle  and  end  of  June, 
and  the  buds  of  the  tertiary  flow^ers  are  then  already  well  developed  in  their 


36  THE   FRUIT   OF   OPUXTIA   FULGIDA. 

upper  areoles.  In  the  latter  part  of  July  flowers  for  the  fourth  generation 
of  the  season  are  developing  from  the  areoles  of  the  tertiary  ones  (fig.  6). 
These  quaternary  flowers  probably  open  during  August,  and  since  the 
blooming-season  may  extend  into  September  (Lloyd,  Plant  World,  1917),  it 
is  probable  that  a  fifth  generation  of  flowers  may  be  formed  in  a  single 
season.  'No  undoubted  examples  of  this,  however,  have  thus  far  been  seen, 
and  I  have  not  been  able  personally  to  seek  them  at  Tucson  at  the  proper 
season. 

Since  the  fruits  resulting  from  the  four  generations  of  flowers  remain 
attached  to  each  other  in  the  order  of  their  development,  it  is  clear  that  a 
chain  of  fruits  of  at  least  four  successive  linlvs  may  be  developed  in  a  single 
season.  This  has  been  recorded  by  Toumey  (1898),  who  apparently 
believed  that  all  chains  of  fruits  arose  in  this  way  in  a  single  season.  He 
definitely  mentions  that  the  "  proliferous  fruit  hangs  in  pendulous  clusters, 
sometimes  as  many  as  7  fruits  in  a  single  cluster,  one  growing  from  the  other 
in  continuous  succession."  Pie  regards  the  persistence  of  fruits  over  winter 
and  the  formation  of  flowers  from  them  in  the  succeeding  spring  as  a  rare 
occurrence. 

The  time  of  maturing  of  the  seeds  of  the  fruits  of  the  four  successive  gen- 
erations may  differ  considerably,  since  the  fruits  continue  to  grow  after 
dropping  the  perianth,  but  even  the  latest  and  smallest  ones  may  contain 
ripe  seeds  in  October.  All  four  links  of  the  chain  may  commonly  persist 
attached  throughout  the  winter  and  give  rise  to  new  flowers  the  following 
spring.  ISTot  infrequently,  however,  the  younger,  quaternary  fruits  may 
wither  more  or  less  during  the  winter  and  (failing  to  develop  flowers  upon 
them  in  the  succeeding  spring)  may  be  crowded  off  by  the  new  flowers 
developed  beside  them  on  the  tertiary  fruit  by  which  they  themselves  are 
borne. 

The  formation  of  these  chains  of  fruits  of  four  links  in  a  single  season 
is  thus  the  result  of  the  continuous  uninterrupted  development  of  the  grow- 
ing-points of  certain  areoles  of  each  successive  floral  shoot.  The  structure 
thus  formed  is  comparable  with  that  developed  by  many  herbaceous  plants 
and  the  water-shoots  of  certain  woody  ones,  in  which  each  (or  many)  of  the 
axillary  buds  develops  continuously  during  the  season  of  its  initiation  into  a 
mature  vegetative  or  reproductive  shoot. 

Other  areoles  of  the  flower  and  fruit  of  this  opuntia  have  quite  an  opposite 
fate.  In  these  the  axial  bud  does  not  develop  continuously,  but  (  after  devel- 
oping a  considerable  number  of  trichomes  and  spicules  and  a  few  nectaries) 
rests  till  the  next  growing-season,  or  sometimes  for  three  or  four  seasons, 
before  renewing  its  development. 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  37 

PROLIFERATION  OF  PERSISTENT  ATTACHED  FRUITS. 

While  certain  of  the  upper  areoles  of  a  flower  may,  as  we  have  seen, 
develop  at  once  into  secondary  flowers  which  open  soon  after  the  primary 
one,  other  areoles  of  the  same  or  another  flower  may  practically  cease  growth 
with  the  opening  of  the  flower  and  persist  over  one  or  more  winters  as  resting 
buds.  These  buds,  retaining  their  capacity  for  further  development,  may 
give  rise,  in  the  succeeding  or  a  later  spring,  to  the  primary  flowers  of 
that  season  (fig.  4,  7c).  Such  flowers  may  arise  from  any  of  the  4  or  5 
younger  fruits  of  a  chain  (fig.  4). 

The  development  of  these  flowers,  arising  from  resting  buds  of  persistent 
fruits,  is  identical  with  that  of  flowers  arising  from  the  areoles  of  flower- 
buds,  and  the  fruits  formed  in  the  two  cases  can  not  be  distinguished.  The 
flowers  arising  from  fruits,  just  as  those  from  the  areoles  of  unopened 
flowers,  may  give  rise  in  turn  to  buds  of  secondary  flowers,  and  these  to  ter- 
tiary and  perhaps  quaternary  ones  in  the  same  season.  The  repetition  of 
this  process  of  budding  out  flowers  from  fruits  and  then  flower  from  flower 
several  times  each  season,  when  repeated  season  after  season,  results  finally 
in  the  formation  of  fruit  clusters  of  great  size.  Clusters  of  20  to  30  fruits 
are  common,  and  clusters  of  100  or  more  fruits  of  all  ages  suspended  from  a 
single  parent  fruit  are  not  rare  (figs.  2,  3).  Some  of  the  longer  chains  may 
embody  12  or  14  generations  of  fruits  in  a  single  chain  (fig.  77).  Since,  as 
we  have  seen,  certain  of  the  links  added  in  any  one  season  may  wither  and 
fall  off,  it  is  evident  that  a  chain  of  12  generations  of  fruits  does  not  repre- 
sent merely  the  growth  of  three  seasons,  4  fruits  per  season,  but  may  repre- 
sent the  product  of  five  or  six  seasons ;  in  fact,  the  size  and  appearance  of 
some  of  the  basal  fruits  of  these  clusters  indicates  an  age  of  more  than  6 
years.  The  records  of  the  effect  of  exceptionally  cold  winters  on  the  plants  at 
Tucson  show  that  it  must  often  take  many  seasons  to  build  up  a  chain  of  12 
or  14  links.  Thus,  in  the  winter  of  1912-13  there  was  an  exceptionally  hard 
freeze,  soon  after  which  great  numbers  of  the  younger  persisting  fruits  fell 
off  the  trees,  so  that  chains  of  more  than  3  or  4  links  were  hard  to  find. 
Such  a  periodic  shortening  of  the  chains,  even  if  it  occurred  but  once  in 
three  or  four  winters,  would  increase  considerably  the  number  of  summers 
necessary  to  produce  chains  of  fruits  of  the  total  length  of  the  longer  chains 
found. 

The  ease  with  which  these  fruits  may  be  set  free  from  the  plant  will  be 
the  more  readily  realized  when  we  note  the  very  slender  stalks  by  which  the 
heavy  fruits  are  attached  (figs.  25,  26).  These  stalks  also,  especially  in  the 
younger  fruits,  have  little  lignified  tissue.  Jarring  of  the  tree  by  A\T[nd  or 
by  broAvsing  cattle,  which  eat  many  of  the  fruits  in  dry  seasons,  may  shake 
off  numbers  of  fruits  that  are  often  found  strewn  thickly  beneath  larger 
plants.  The  stalks  of  the  older  fruits,  on  the  contrary,  become  steadily 
thicker  and  stronger,  and  the  upper  ones  are  thereby  enabled  to  hold  the 
heavy  clusters  that  hang  from  them. 


38  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

PROLIFERATION  OF  FALLEN  FRUITS. 

If  the  fruit  of  Opuntia  fulgida  remains  attached  to  the  tree  the  only  struc- 
tures produced  by  its  areoles  are  trichomes,  nectaries,  and  flowers.  This  is 
practically  universally  true.  Among  many  hundreds  examined,  only  two 
cases  were  seen  in  which  a  vegetative  shoot  had  arisen  from  an  attached 
fi-uit.  If,  however,  fruits  are  plucked  from  the  tree  and  placed  on  moist 
soil,  there  arise  from  their  areoles  not  flowers  but  roots  and  later  vegetative 
shoots,  and  so  new  plants  are  initiated ;  that  is,  if  any  given  fruit  remains 
attached  there  may  arise  from  the  growing-point  of  a  certain  areole  a  rela- 
tively short  shoot  which  develops  leaves  and  axillary  buds,  but  soon  ends  its 
activity  with  the  formation  of  a  set  of  stamens  and  carpels.  If  the  same 
fruit  were  detached  the  same  areole,  and  thus  the  very  same  growing-point, 
may  give  rise  to  a  shoot  of  unlimited  growth,  while  other  areoles  near  the 
soil  form  adventitious  roots.  This  production  of  new  plants  from  buds  on 
the  wall  of  the  ovary  in  the  fruit  is  a  surprising  phenomenon ;  in  fact,  it  is  as 
unique  as  the  formation  of  flowers  from  axillary  buds  of  the  ovary  of 
the  unopened  flowers. 

This  process  of  the  vegetative  sprouting  of  a  fruit  planted  in  soil  to  a  new 
plant  often  occurs  in  50  to  75  per  cent  of  fruits  planted  in  the  greenhouse. 
In  the  field  about  Tucson,  at  least  in  the  spring  of  the  year,  it  occurs  but 
rarely.  A  careful  examination  of  some  scores  of  young  plants  of  this 
species,  of  3  or  4  joints  in  height,  in  the  desert  near  Tucson,  in  May  1915, 
showed  that  all  had  arisen  from  fallen  vegetative  joints.  ISTone  of  the  fallen 
fruits  seen  at  this  time  showed  any  preparation  for  the  development  of  new 
plantlets.  This  is  the  more  surprising  because  the  soil  had  been  considerably 
moistened  that  season  by  unusually  copious  rains.  In  September,  however, 
a  careful  search  by  an  assistant,  B.  R.  Bovee,  revealed  a  few  very  young- 
plants  which  had  evidently  arisen  from  fallen  fruits.  It  is  possible,  there- 
fore, that  under  some  conditions,  such  as  those  existing  during  years  of 
favorable  summer  rains,  a  considerable  number  of  new  plants  may  thus  arise 
from  fruits. 

The  origin  of  new  plants  from  rooted  fruits  will  be  described  as  it  has  been 
observed  in  the  greenhouse.  The  process  is  clearly  the  same  in  the  field,  as 
far  as  could  be  seen  from  the  few  plantlets  found  there.  Many  different 
plantings  of  the  fruits  were  made  at  Tucson  in  April  and  May,  at  South 
Harpswell  in  July  and  August,  and  at  Baltimore  in  February,  October,  and 
December,  These  all  gave  substantially  the  same  results,  in  spite  of  the  fact 
that  the  Tucson  plantings  were  of  fruits  that  would  in  a  week  or  two  have 
produced  flowers  if  they  had  been  left  on  the  parent  plant,  while  those 
planted  in  Baltimore  had  entered  into  the  resting-stage  for  the  winter  (figs. 
78,99). 

Fruits  that  are  half -buried  in  a  soil  that  is  kept  moist  but  not  saturated 
may  begin  to  form  adventitious  roots  within  the  first  week.  In  5  weeks'  time 
the  fruit  has  been  found  fastened  securely  in  the  soil  by  several  roots,  some 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  39 

of  them  5  cm.  long,  1  mm.  thick,  and  often  branched  several  times.  These 
roots  arise  chiefly  on  the  buried  portion  of  the  fniits  and  always  develop 
from  an  areole,  from  the  broken  surface  of  the  fruit-stalk,  or  more  rarely 
from  near  the  edge  of  the  perianth-scar.  From  this  statement  it  will  be 
seen  that  roots  may  arise  at  either  the  basal  or  apical  end  of  the  fruit,  or 
at  both,  depending  on  whether  the  fruit  is  laid  horizontally  or  whether  one 
end  is  placed  lower  in  the  soil.  My  observations  do  not  confirm  the  con- 
clusion of  Toumey  (1905)  that  the  "  roots  appear  chiefly  at  the  basal  end  of 
the  fallen  joint."  Though  the  roots  arise  from  the  areole,  they  do  not  arise 
from  its  growing-point,  in  the  middle  of  the  areole,  but  from  the  still  active 
tissue  around  its  edge.  Very  often  a  root  pushes  out  above  the  cluster  of 
bristles  on  the  adaxial  margin  of  the  areole,  but  a  root  may  rise  also  from 
any  other  part  of  the  margin  (fig.  100).  One  or  several  roots  may  arise 
from  the  same  areole.  When  a  root  is  developed  from  the  scar  of  the  fruit- 
stalk  it  is  usually  from  its  margin.  As  such  a  root  matures,  however,  the 
vascular  bundles  of  the  root  soon  come  to  form  continuations  of  the  bundles 
at  the  base  of  the  fruit.  When  a  root  develops  on  a  perianth-scar  it  appears 
always  to  originate  in  the  region  of  the  cork  cambium,  and  it  breaks  through 
the  cork  itself  in  emerging  (figs.  78,  99,  100). 

The  initiation  of  a  shoot  by  a  detached  fruit  does  not  occur  until  some 
time  after  the  first  roots  are  developed  on  it.  The  formation  and  function- 
ing of  roots  are  apparently  necessary  antecedents  to  shoot  formation.  In 
fruits  planted  in  the  greenhouse  at  Tucson  on  April  26,  kept  well  watered 
and  at  rather  high  temperatures,  shoots  began  to  push  out  of  some  areoles  by 
May  25,  and  on  the  following  September  10  all  but  one  or  two  of  the  18 
fmits  planted  had  one  or  more  vegetative  joints  on  it.  Some  of  the  latter 
were  2  cm.  long.  Similar  results,  though  not  so  universally  successful,  were 
obtained  from  plantings  at  South  Harpswell  and  Baltimore.  One  lot  of 
fruits  planted  at  Baltimore  produced  shoots  only  3  or  4  cm.  long  in  a  year ; 
while  others,  with  more  soil,  had  shoots  10  cm.  long  in  five  months. 

One  or  several  joints  may  arise  from  each  fruit,  either  simultaneously  or 
successively.  They  may  arise  from  areoles  on  the  exposed  surface  of  the 
planted  fruit  or  sometimes  from  those  joints  at  the  surface  of  the  soil,  but 
nearly  always  from  the  larger  areoles  of  the  apical  half  of  the  fruit.  This  is 
without  regard  to  whether  the  fruit  is  planted  on  the  side,  -with  apex  down, 
or  with  the  base  do\\ai. 

The  shoot  arising  from  the  areole  of  a  fallen  fruit  is  formed  by  the  grow- 
ing-point of  the  areole,  just  as  a  flower  is,  or  just  as  a  branch  is  from  a  vege- 
tative joint.  The  new  shoot,  however,  is  for  a  long  time  dift'erent  from  the 
branch  of  a  mature  plant  in  remaining  slender  and  in  the  permanent  delicacy 
of  its  spines  (fig.  78).  In  fact,  it  has  the  appearance  of  one  of  the  early 
joints  of  a  seedling.  The  primary  shoot,  as  developed  in  the  greenhouse  in 
Baltimore,  usually  does  not  branch  until  the  second  season.  Quite  early  in 
its  development  the  new  shoots,  especially  if  it  has  arisen  near  or  under  the 


40  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

soil,  sends  down  adventitious  roots  of  its  own  which  soon  become  an  impor- 
tant part  of  the  root-sjstem  of  the  new  plant. 

From  what  has  been  said  of  the  slow  development  of  plantlets  from 
sprouting  fruits  in  the  laboratory,  it  is  evident  that  it  takes  a  number  of 
years  to  develop  a  mature  flowering  plant  from  a  fruit.  It  is  doubtful 
whether  such  a  plant  can  mature  more  than  a  year  or  two  sooner  than  a  seed- 
ling started  at  the  same  time. 

Examination  of  plantlets,  from  sprouted  fruits  and  also  of  those  from 
fallen  joints,  shows  that  not  all  of  the  fallen  fruit  or  joint  enters  into  the 
make-up  of  the  first  or  basal  joint  of  the  new  plant.  The  portion  which  does 
so  probably  depends  in  part  upon  the  position  in  which  the  fruit  falls,  or  is 
planted,  on  the  soil  and  on  the  relative  positions  on  the  fruit  of  the  areoles 
forming  the  roots  and  those  forming  the  shoots.  It  is  a  common  occurrence 
for  a  part  (often  a  fourth  or  a  third)  of  the  fruit,  including  both  cortex  and 
vascular  bundles,  to  be  cut  off  by  a  layer  of  cork  from  the  part  going  into 
the  first  joint  of  the  new  plant  (fig.  99).  The  phellogen  from  which  this 
cork  arises  is  formed  by  the  parenchyma  of  the  cortical  and  medullary- 
tissues. 

The  fate  of  the  different  parts  of  the  vascular  system  of  the  parent  fruit 
has  not  been  followed  out  in  all  details,  but  it  is  clear  that  much  of  the  old 
system  plays  no  important  part  in  the  new  plant  and  that  some  of  it  is  cast 
off  with  the  cut-off  portion  of  the  fruit  (fig.  99).  The  chief  part  of  the  old 
system  to  do  service  in  the  new.plantlet  is  that  which  lies  most  directly 
between  the  point  of  origin  of  the  adventitious  roots  and  that  of  the  primary 
shoot  of  the  plantlet  (fig.  99).  These  strands  quickly  become  thickened  to 
many  times  the  diameter  of  the  other  bundles  of  the  parent  fruit.  The  seeds 
present  in  these  sprouting  fruits  evidently  persist  indefinitely  in  the  flesh  of 
the  latter.  Their  ultimate  fate  has  not  yet  been  determined,  as  the  oldest 
plantlets  seen  which  were  known  to  be  of  this  origin  were  but  2  years  old. 

While  roots  may  arise  from  parts  of  the  fallen  fruit  outside  the  areole, 
such  as  the  scar  of  the  fruit-stalk  or  of  the  perianth,  it  is  apparently  not  pos- 
sible for  shoots  to  arise  elsewhere  than  from  the  areole.  The  experiment 
was  tried  repeatedly  of  planting  fruits,  all  the  areoles  of  which  had  been 
destroyed  by  cauterization,  to  determine  whether  other  superficial  tissues 
might  be  stimulated  to  produce  new-shoot  growing-points.  Though  some  of 
these  cauterized  fruits  took  root  in  the  soil  and  remained  plump  and  green 
for  24  months,  they  developed  no  visible  rudiment  of  a  shoot. 

An  attempt  was  made  also  to  determine  whether  halves  or  quarters  of  a 
fruit,  in  which  the  cut  surfaces  were  either  covered  with  vaseline  or  dried, 
could  be  made  to  take  root  and  form  new  shoots.  This  was  partially  success- 
ful in  only  a  few  instances  where  part  of  the  piece  remained  green  for  a  time 
and  some  roots  were  formed,  but  in  no  case  was  a  single  shoot  formed.  This 
was  due  apparently  to  the  fact  that  a  softening  and  decay  of  the  exposed  pulp 
of  the  cut  fruit  took  place,  similar  to  that  which  occurred  in  fruits  punc- 
tured for  the  insertion  of  cut  seeds  (see  p.  33). 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  41 

Finally,  attempts  were  made  to  determine  by  experiment  whether  an 
areole  that  had  once  borne  a  fruit  could,  after  careful  removal  of  this,  give 
rise  to  a  vegetative  shoot.  When  from  such  fmits  all  the  flowers  and  sec- 
ondary fruits  Avere  removed,  and  all  other  areoles  cauterized,  and  the  fruits 
then  planted  in  soil,  no  shoots  at  all  arose  from  any  of  their  areoles.  This, 
of  course,  is  rather  to  be  expected,  with  but  a  single  growing-point  in  each 
areole.  But  the  latter  condition  had  not  been  demonstrated  histologically 
at  the  time  the  experiment  was  initiated.  Then,  too,  it  could  not  be  assumed 
impossible  for  some  of  the  younger,  protected  cells  from  about  the  margin 
of  an  areole  of  the  primary  fruit  that  had  already  produced  a  secondary 
fruit  to  give  rise  to  a  new-shoot  growing-point,  just  as  cells  outside  the  areole 
may  initiate  a  new-root  growing-point.  In  a  check  experiment  21  fruits 
that  had  borne  flowers  were  planted  after  the  removal  of  these  and  no  areoles 
cauterized.  In  10  months  one  fruit  was  dead,  21  were  living  and  rooted,  and 
17  of  these  bore  shoots  3  to  4  cm.  high.  This  demonstrated  that  no  wound 
injurious  to  the  fruit  as  a  whole  is  caused  by  the  removal  of  the  secondary 
flower  or  fruit.  On  the  other  hand,  the  fact  that  many  of  the  cauterized 
fruits  mentioned  took  root  and  remained  green  and  plump  for  two  years 
demonstrates  that  cauterization  does  not  seriously  injure  the  fruit  as  a  whole. 

CAUSES  AND  SIGNIFICANCE  OF  PERENNATION  AND  OF  THE 
DIVERSE  TYPES  OF  PROLIFERATION. 

We  have  described  above  the  unusual  persistence  and  secondary  growth  of 
the  fruits  of  this  opuntia  and  the  entirely  unique  power  of  the  proliferation 
to  flowers  and  vegetative  shoots  shown  by  the  areoles  of  its  ovary.  Beyond 
these  facts  of  the  structure  and  habit  of  fruit  and  areole  lie  the  physiological 
problems  of  the  causes  and  significance  in  the  life-history  of  these  striking 
peculiarities  in  this  species  of  Opuntia. 

The  primary  question  to  be  answered  is:  Why  does  the  fruit  of  this 
cactus  never  ripen  like  those  of  most  cacti  and  of  the  vast  majority  of  other 
angiosperms  ?  ITo  appreciable  light  on  this  question  has  been  obtained  from 
a  study  of  this  opuntia  itself.  Not  a  single  fruit  of  this  species  has  been 
seen,  not  even  an  abnormal  one,  that  showed  any  indication  of  undergoing 
those  changes  which  characterize  the  process  of  ripening  in  most  other  cacti. 

The  persistence,  year  after  year,  of  fruits  containing  mature  ripe  seeds  in 
attachment  to  the  parent  plant  is  another  peculiarity  entirely  unexplained 
by  observation  thus  far  made  on  this  opuntia.  Dropping  off  or  springing 
open  is  in  the  case  of  most  plants  a  phenomenon  closely  associated  with  the 
process  of  ripening.  The  most  noteworthy  exception  to  this  general  rule  is 
that  of  Callistemon  and  allied  Myrtaceje  investigated  by  Ewart  (1907), 
where  the  fruits  persist  almost  indefinitely  until  killed  by  the  cutting  off  of 
the  water-supply  by  fire  or  drought  (fig.  80). 

One  possible  explanation  of  the  persistent  greenness  and  attachment  of 
the  fruit  is  suggested  by  the  study  of  an  interesting  abnormal  phenomenon 


42  THE   FRTJIT   OF   OPUNTIA   FULGIDA. 

observed  in  certain  other  Arizona  opimtias.  The  most  striking  case  of  those 
studied  is  afforded  by  Opwifia  versicolor^  a  species  in  which  the  normal 
structure  of  shoot,  flower,  and  fruit  is  quite  similar  to  that  found  in  Opuntia 
fulgida. 

The  fruit  of  0.  versicolor  is  quite  variable  in  form,  size,  and  habit  as  to 
ripening  and  persistence.  It  may  be  nearly  globular  and  from  15  to  20  mm. 
in  diameter,  as  is  true  of  most  of  the  smaller  fruits,  or  it  may  be  a  much-elon- 
gated structure  whose  whole  length  is  4  or  5  times  its  diameter  (figs.  82,  84). 
These  fruits  do  not  soften  greatly  or  change  color  with  the  ripening  of  the 
seeds,  but  remain  green  or  yellowish  during  the  autumn  and,  according  to 
Tourney  (1898),  usually  ripen,  wither,  and  dry  up  while  still  on  the  tree 
during  early  winter.  Some  of  the  apparently  normal  fruits,  however,  as 
Toumey  noted,  may  remain  attached  for  a  year  or  even  two  years.  This  is 
demonstrated  by  figures  82,  83,  and  86,  which  were  photogi-aphed  in  late 
April. 

On  examination  of  large  numbers  of  these  plants  of  Opuntia  versicolor  in 
April  and  May,  it  was  found  that  most  of  them  (about  Y5  per  cent)  bore  no 
persistent  fruits.  Of  those  plants  which  did  bear  apparently  normal  per- 
sistent fruits,  9  out  of  10  bore  abnormal  gall-like  fruits  also,  of  which  we 
shall  say  more  presently.  It  seems  possible  then,  that  the  cause  of  the  per- 
sistence of  the  normal  fruits  may  be  the  same  as  the  cause  of  the  abnormality 
as  well  as  of  the  persistence  of  the  far  more  common  gall  fruits. 

These  gall  fruits  have  an  exceedingly  interesting  developmental  history. 
They  seem  very  clearly  to  be  caused  by  the  deposit  in  the  flower-buds  of  the 
eggs  of  the  cactus  fly  {Asphondylia  opuntice).  These  eggs  may  apparently 
be  deposited  at  different  times  in  different  cases,  for  the  galls  show  that  the 
flower  bud  has  been  arrested  and  diverted  from  its  normal  course  at  different 
phases  of  its  development.  The  galls  show  that  in  some  cases  the  arrest  of 
normal  development  of  the  flower  occurred  when  the  perianth  had  hardly 
been  initiated,  in  other  cases  after  the  perianth  had  been  half -formed,  and 
in  yet  others  not  until  the  perianth  was  fully  developed  (fig.  84,  a,  h,  c,  d). 

The  degree  of  disturbance  of  the  normal  development  of  the  internal 
organs  of  the  flower  differs  markedly.  In  some  cases  the  stamens,  pistils, 
and  even  the  ovules  may  have  been  well  started  only  to  become  distorted  and 
withered  without  maturing,  while  in  others  no  traces  of  stamens  or  ovules 
are  te  be  discovered.  Quite  independent  of  these  internal  features  are  the 
size  and  external  form  attained  by  the  distorted  bud.  Sometimes  it  may  be 
no  larger  than  some  normal  buds  at  the  time  of  opening,  while  again  it  may 
reach  a  length  and  diameter  twice  or  thrice  the  normal  (fig.  84  a,  c).  In 
general  external  appearance  most  of  these  fruits  in  early  April  have  a  plump 
green  ovary  and  often  dark-red,  waxy  petals.  Many  of  them,  in  fact,  look 
precisely  like  gigantic,  but  otherwise  normal  flower  buds  that  seem  just 
ready  to  burst  into  bloom  (fig.  84(i).  It  is  surprising  that  the  petals  can 
persist  all  winter  and  retain  a  color  which  though  darker  than  the  outside  of 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  43 

the  petals  is  not  very  different  from  that  of  the  inner  surface  of  the  petals  in 
many  flowers  of  this  cactus.  A  section  through  such  an  abnormal  flower-bud 
gall  taken  in  April  shows  from  a  few  to  dozens  or  sometimes  scores  of  the 
small  pupge  of  Asphondylia  within  the  gall.  They  are  embedded  in  the 
cortical  region  of  the  ovary,  chiefly  in  the  portion  above  the  small  ovarian 
cavity,  and  they  stand  perpendicular  to  the  surface  of  the  bud. 

In  spite  of  the  rather  normal  appearance  of  many  of  the  flower-buds  which 
have  wintered  over  unchanged  from  the  preceding  season,  they  never  open  to 
flowers.  The  nearest  approach  to  this  process  is  found  in  the  curling  open 
of  the  tips  of  the  petals  as  the  buds  finally  wither  (fig.  81).  The  only  open- 
ing of  these  galls  that  does  occur  is  of  quite  a  different  sort.  During  May, 
especially  in  the  latter  half,  the  pupse  of  the  cactus  fly  transform  to  imagos 
and  break  out  through  the  epidermis  of  the  bud,  each  independently.  The 
pupa-cases  are  left  with  half  their  length  projecting  beyond  the  surface  of 
the  gall  (fig.  81).  The  fly  itself  perches  on  the  gall  while  its  wings  are 
hardening  (figs.  81,  84),  and  then  flits  off  to  play  its  part  in  infecting  the 
new  flower-buds  of  the  season,  which  are  then  just  pushing  out  of  the  areoles 
of  the  vegetative  joints.  By  the  end  of  May  a  large  share  of  the  emptied 
galls  have  withered  and  dropped  to  the  ground,  where  they  decay,  often  by 
the  aid  of  fungi,  which  have  ready  entrance  about  the  old  pupa  cases. 

We  come  now  to  the  consideration  of  the  possible  relation  between  the 
gall  fruits  or  gall  buds  and  the  apparently  persistent  fruits.  As  was  stated 
above,  many  of  these  latter  fruits  contain  apparently  good  seeds.  Others 
have  only  withered  rudiments  of  seeds,  and  thus  resemble  certain  of  the 
gall  fruits.  In  fact,  a  complete  series  of  structures  can  be  discovered 
grading  almost  imperceptibly,  in  both  external  and  internal  features,  from 
typical  fruits  to  a  persistent  normal-appearing  flower  bud.  This,  together 
with  the  fact  that  the  normal  type  of  persistent  fruit  in  90  per  cent  of  the 
cases  occurs  on  plants  that  also  bear  gall  fraits,  suggests  that  both  have  a 
common  cause  (figs.  82,  84). 

If  it  is  the  presence  of  the  egg  or  larva  of  Asphondylia  or  of  some  sub- 
stance deposited  with  the  egg  that  inhibits  the  normal  development  of  the 
flower,  but  stimulates  it  to  an  abnormal  and  locally  excessive  growth  and  also 
causes  it  to  persist  over  winter,  then  some  lesser  amount  or  degi-ee  of  this 
same  stimulus  may  be  responsible  for  the  persistence  of  the  apparently  nor- 
mal fruits.  Such  stimulation  might  occur  either  by  transmission  of  some 
substance  or  of  some  stimulus  from  an  infected  fruit  to  a  neighboring  one, 
which  had  not  been  infected.  It  is  also  possible  that  a  fruit  which  had  been 
bored  for  the  deposit  of  a  single  egg,  or  a  few,  might  continue  to  develop 
normally  in  every  respect,  except  that  it  persisted  on  the  tree.  The  only  way 
to  distinguish  between  these  two  possibilities  would  be  by  experimental 
study.  The  latter  of  the  two  \aewt)  stated  seems  supported  by  the  observed 
fact  that  the  buds  containing  most  pupa?  are  generally  the  ones  most  modified 
in  structure,  while  buds  or  fruits  with  few  pupse  are  relatively  little 
modified. 


44  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

In  other  species,  such  as  the  flat- jointed  Opimtia  discata,  the  disturbance 
of  the  normal  development  of  the  flower  and  fruit  hj  the  laying  of  the  eggs 
of  Asphondylia  is  never  so  marked  as  in  Opuntia  versicolor.  The  perianth 
of  this  species  is  always  shed,  leaving  a  definite  scar.  This  probably  means 
that  it  develops  to  maturity  and  then  opens  more  or  less  completely.  The 
general  form  and  size  of  the  resulting  fruit  are  relatively  little  affected  (fig. 
87).  The  ripening  of  the  fniit  also  is  only  partially  inhibited,  though  the 
complete  ripening  and  subsequent  fall  of  the  fruit  are  rarely  accomplished 
normally.  In  one  example  found  at  Tucson  a  fruit  of  1913,  bearing  50  or 
more  Aspliondylia  scars,  persisted  and  bore  a  vegetative  joint  5  cm.  long 
in  1914,  and  both  were  plump  and  green  when  collected  in  May  1915. 
Usually  the  fly  escapes  from  its  pupa-case  and  the  riddled  f niit  mthers  as  it 
does  in  Opuntia  versicolor. 

Griffiths  (1913,  p.  18)  reports  that  a  similar  retention  of  the  gall-like 
flower-buds  or  fruits  in  Opuntia  puberula  is  due  to  the  attack  of  the  black 
opuntia  louse. 

If  the  explanation  suggested  above  for  the  retention  and  modification  in 
structure  of  the  flower-buds  of  Opuntia  versicolor  is  the  real  one  for  this 
case,  then  it  is  quite  possible  that  a  similar  one  may  be  found  for  the  similar 
peculiarities  of  0.  fulgida.  That  is,  it  is  conceivable  that  some  relatively 
slight  change  or  lack  of  change  in  the  nature  of  the  cell-sap,  or  in  the  char- 
acter of  photosynthetic  or  other  metabolic  products  of  Opuntia  fulgida,  from 
a  cause  as  simple  as  the  sting  of  a  fly,  though  as  yet  undiscovered,  may  prove 
the  adequate  explanation  of  the  peculiarities  of  the  fruit  of  this  cactus. 
Experiments  have  been  initiated  to  test  this  hypothesis  by  the  aid  of  injec- 
tions and  by  otherwise  changing  the  external  or  internal  conditions  affecting 
the  plant.     They  have  not  yet  been  completed. 

We  have  still  to  discuss  the  marked  diversity  in  behavior,  under  the  same 
conditions,  of  the  different  axillary  buds  of  the  flower  or  matured  fruit,  and 
also  that  of  the  same  buds  of  the  fruit  under  different  conditions.  If  a 
mature  fruit  of  Opuntia  fulgida  is  taken  from  the  tree  at  any  time  from 
October  to  April  and  placed  on  damp  soil,  some  of  its  areoles  will  push  out 
to  vegetative  shoots.  On  the  other  hand,  if  the  same  fruit  were  to  be  left  on 
the  tree  till  May  no  shoots  at  all  would  be  formed,  but  the  same  areoles  might 
give  rise  to  flowers  instead  of  vegetative  shoots.  Moreover,  while  only  the 
areoles  of  the  distal  quarter  of  the  flower  or  fruit  develop  so  long  as  the  fruit 
remains  attached,  any  of  the  buds,  except  those  of  the  very  basal  quarter  of 
the  fallen  fruit,  may  give  rise  to  root  or  shoot.  The  areoles  which  actually 
do  this  are  determined  by  the  position  of  the  fruit  on  the  soil.  Finally,  if 
these  same  fruits  are  taken  from  the  tree  about  May  1  and  planted,  the 
areoles  which  had  already  begun  to  swell  slightly  do  not  go  on  to  develop  vege- 
tative shoots,  but  may  either  fail  altogether  to  develop  or  give  rise  to  small, 
imperfect  flower-buds,  which  soon  wither  and  drop  off.  Later,  other  and 
more  basal  areoles  may  proliferate  to  vegetative  shoots. 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  45 

The  questions  we  have  to  answer  here  are  these :  Why  do  areoles  of  fruits 
that  are  picked  and  planted  in  April  give  rise,  in  the  follovs^ing  month,  to 
nothing  but  vegetative  shoots  when  if,  on  the  other  hand,  these  fruits  were 
to  be  left  attached  till  May  the  same  areoles  would  give  rise  in  that  month  to 
flowers,  and  to  these  only  ?  Secondly,  what  sort  of  change  occurs  in  the 
areoles  or  fruit  between  April  1  and  May  1  which  makes  the  fruit  detached 
at  the  latter  date  incapable  of  doing  what  it  could  do  if  detached  at  the  earlier 
one  ?  Thirdly,  why  do  none  but  the  most  distal  buds  of  a  fruit  give  rise  to 
flowers,  while  any  but  the  most  basal  areoles  of  a  fallen  fruit  may  develop 
shoots  ? 

In  regard  to  the  first  question,  it  has  occurred  to  the  writer  that  the 
attached  fruits  give  rise  to  flowers  because  these  fruits  and  their  areoles  are 
supplied  with  substances — perhaps  flower-forming  substances — which  differ 
markedly  from  those  supplied  to  the  areoles  of  the  same  fruit  when  it  has 
fallen  and  become  rooted  in  the  ground.  Or,  perhaps  the  process  of  root 
formation  may  itself  produce  substances  that  inhibit  flower  formation  in  the 
same  fruit.  The  plan  to  experiment  on  this  by  rooting  attached  fruits  in 
pots  of  soil  supported  beneath  them  in  the  field  has  not  yet  been  carried  out. 
Several  attached  vegetative  joints  of  a  plant  growing  in  the  greenhouse  at 
Baltimore,  which  from  their  position  seemed  likely  to  produce  flowers,  were 
rooted  in  pots  placed  beneath  them.  In  the  first  season,  however,  these  joints 
developed  neither  shoots  nor  fruits.  It  is  planned  to  repeat  this  experiment 
on  attached  fruits  and  joints  in  the  field  at  the  first  opportimity. 

When  a  good-sized  vegetative  joint  or  two  consecutive  joints  bearing  sev- 
eral fruits  is  rooted  in  the  soil,  these  fruits  still  give  rise  to  shoot-buds  only. 
This  is  true  in  spite  of  the  fact  that  tlie  conditions  of  nutrition  here  would  be 
expected  to  be  more  nearly  like  those  of  fruits  on  a  growing  plant. 

It  was  also  attempted  to  discover  by  experiment  whether  the  production  of 
flowers  alone  by  attached  fruits  is  definitely  influenced  by  the  amount  and 
kind  of  nutritive  material  available  for  them,  in  consequence  of  their  rela- 
tion to  the  vegetative  branches  bearing  them  and  to  other  fruits.  In  each  of 
4  plants  of  Opuntia  fulgida  several  branches  were  denuded  of  all  fruits 
except  3  or  4  sets  of  1  or  2  fruits  each.  After  three  seasons'  growth  the  num- 
ber of  new  flowers  and  fruits  that  had  arisen  from  these  undisturbed  fruits 
was  not  at  all  abnoraially  increased,  nor  had  the  treatment  induced  the  for- 
mation of  a  vegetative  joint  on  a  single  one  of  the  original  fruits.  The  chief 
tendency  of  the  new  growth  in  these  plants  was  toward  the  development  of 
new  vegetative  joints  from  the  old,  partially  denuded  ones.  This  was  most 
strikingly  shown  in  a  tree  from  which  300  fruits  and  100  joints,  including 
the  top  of  the  stem,  had  been  lopped  off  and  only  two  fertile  branches  with 
small  fruit-clusters  left.  In  this  tree  the  new  growth  consisted  almost 
entirely  of  vegetative  joints  clustered  about  the  cut  ends  of  the  main  stem 
and  branches.     In  these  cases,  therefore,  the  diversion  of  all  available  nutri- 


46  THE   FETJIT   OF   OPUNTIA   FULGIDA. 

tive  material  of  the  vegetative  branch  into  one  or  two  fruits,  instead  of  into 
several  scores,  did  not  change  the  fate  of  the  areoles  on  these  fmits.  There 
was  no  increase  of  the  vegetative  activity  in  them  such  as  might  have  been 
expected  as  a  result  of  the  increased  available  food-supply. 

In  seeking  a  reply  to  the  second  question  proposed  on  page  45  it  was 
found  that  all  attempts  made  by  planting  very  young  flower-buds  to  induce 
these  to  become  metamorphosed  into  vegetative  shoots  were  unsuccessful. 
The  evidence  from  these  experiments  seemed  to  show  that  when  once  tha 
growing-point  of  an  areole  has  started,  even  if  but  barely  started,  to  form  a 
flower,  it  can  not  be  diverted  and  made  to  give  rise  to  a  vegetative  shoot,  as  it 
could  by  removing  the  fruit  from  the  plant  and  planting  it  a  few  weeks 
earlier.  In  every  case  the  young  flower-bud  on  a  planted  fruit  either  failed 
to  develop  at  all  or  developed  but  slightly  and  then  withered. 

In  essential  agreement  with  these  results  in  sprouting  the  detached  fruits 
are  those  observed  in  detached  vegetative  joints  of  Opuntia  fulgida.  Fallen 
joints,  as  we  have  noted,  very  commonly  take  root  and  give  rise  to  new  plants. 
In  no  case  of  scores  observed  were  flowers  developed  from  such  rooted 
joints  until  after  a  considerable  shoot  system  had  been  formed. 

In  Opuntia  vulgaris,  however,  a  series  of  experiments  made  in  May  and 
June  1917,  gave  very  different  results.  In  practically  every  case  the  term- 
inal joint  or  pair  of  joints  removed  just  before  the  flower  buds  appear,  or 
just  after  they  are  visible,  will  root  promptly  and  then  develop  normal  flowers 
and  in  most  cases  set  fruit  (see  Hildebrandt,  1888,  p.  110). 

The  third  question  raised  must  apparently  be  answered  by  attributing  to 
the  influence  of  polarity  the  restriction  of  flower  buds  to  the  most  distal 
areoles  of  the  fruit.  This  is  very  pronounced  so  long  as  the  fruit  is  attached, 
and  is  definitely  related  to  the  base ;  that  is,  to  the  point  of  attachment  of  the 
fruit.  In  fallen  fruits  a  much  less  definite  polarity  is  exhibited  which  is 
determined  by  the  points  of  origin  of  roots  and  new  shoots. 

Another  problem  suggesting  itself  in  connection  with  the  production  of 
flowers  from  fruits  is  the  discovery  of  the  reason  for  the  fact  that  only  the 
terminal  fruits  of  a  cluster  and  a  few  of  the  subterminal  ones  give  rise  to 
fruits,  while  all  the  fruits  basal  to  these,  from  1  to  8  or  even  10,  produce  no 
flowers.  This  seemed  to  be  equally  true  whether  this  basal  part  of  a  chain 
bore  one  or  several  secondary  or  branch  chains  upon  it.  In  collaboration 
with  Dr.  Hermann  Spoehr,  who  planned  the  chemical  side  of  the  work, 
analyses  were  made  of  the  pulp  of  the  two  basal  fruits  and  the  two  terminal 
fruits  of  each  of  several  chains  of  six  or  more  fruits.  We  were  unable,  how- 
ever, to  discover  any  difference  in  the  carbohydrates  and  other  nutrient  sub- 
stances present  in  the  distal  flower-forming  fruits  and  the  basal  sterile  ones. 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  47 

PROLIFERATION  OF  THE  FLOWER  OR  FRUIT 
IN  ALLIED  SPECIES. 

The  proliferation  of  the  ovarian  wall,  either  in  flower  or  fruit,  has  been 
noted  in  a  number  of  other  opimtias,  and  in  at  least  one  other  genus,  by 
Engehnaun  (1887),  Hildebrandt  (1888),  and  a  number  of  more  recent 
workers.  (See  also,  Penzig,  1890,  p.  507).  Associated  with  this  prolif- 
eration in  certain  cases  we  find  a  persistence  of  the  fruits  for  one  or  more 
years  after  maturing.  The  areoles  of  the  attached  fruits  of  some  of  these 
species  are  known  to  form  flowers  only.  The  attached  fruits  of  others,  on 
the  contrary,  may  develop  not  only  new  flowers  and  fruits,  but,  under  certain 
conditions,  give  rise  to  vegetative  shoots  also.  Of  the  first  type  are  Opuntia 
cylindnca,  0.  leptocaulis,  0.  catacantha,  and  Peireskia  guamacho  Rose. 
Of  the  second  sort  are  Opuntia  rufida,  0.  spinosissima ,  0.  discata,  0.  versi- 
color, and  0.  arbuscula. 

Opuntia  cylindrica,  growing  under  cultivation  but  out  of  doors  at  Del 
Monte,  California,  frequently  formed  flowers  by  proliferation  from  the  per- 
sistent fruits  of  the  previous  season  (fig.  88).  This  same  species  also  fur- 
nished striking  examples  of  the  development  of  first  a  vegetative  joint  and 
then  a  fruit  by  the  uninterrupted  activity  of  the  same  growing-point ;  that  is, 
the  joint  and  fruit  are  separated  by  only  a  very  slight  constriction,  as  was 
noted  in  speaking  of  Opuntia  fulgida  (cf.  figs.  7c^  88).  From  the  plants  of 
0.  cylindrica  observed  there  is  no  evidence  that  the  primary  flowers  ever  give 
rise  to  buds  of  secondary  ones  before  they  are  open. 

Opuntia  toumeyi,  growing  near  Tucson,  may  occasionally  form  secondary 
flowers  close  to  the  base  of  the  primary  ones,  which  open  soon  after  the  latter. 

Opuntia  leptocaulis  is  a  slender  Arizona  species,  growing  along  with  0. 
fulgida,  in  which  fruits  may  persist  unripened  or  half-ripened  and  then 
bud  out  new  flowers  in  the  succeeding  season  (fig.  89,  a,  h,  c).  These  pri- 
mary fruits  may  persist  for  a  year  and  a  half  and  the  secondary  ones  may 
ripen,  but  tertiary  fruits  are  rarely  formed.  The  propagative  structures 
here  show  a  very  closely  graded  series  of  intenuediatc  forms  between  the 
typical  fruit  and  the  vegetative  joint,  a  series  far  more  complete  than  can 
usually  be  found  on  0.  fulgida  or  any  other  near  Tucson.  One  plant  of  0. 
leptocaulis  seen  at  Chico  in  August  bore  numerous  slender  vegetative 
joints  on  persistent  fruits  of  the  same  season  and  also  of  the  preceding  season. 

The  graded  series  of  propagative  structures  above  mentioned  contains 
typical  obovate  fruits,  with  definite  perianth-scars,  evidently  fonned  by  nor- 
mally opened  flowers.  Other  fruits  are  twice  as  long,  but  still  show  the  scar 
of  a  normal  perianth.  Then  there  are  joints  of  various  lengihs,  of  wliich 
some  bear  small  or  very  small  perianth-scars  while  others  have  no  scars  at 
all,  and  yet  all  of  them  look  very  fruit-like  in  other  external  aspects.  There, 
are  also  many  slender  joints  having  no  perianth-scars,  yet  closely  resembling 
the  more  slender  sterile  fruits  that  do  have  them,  l^one  of  these  various 
structures  except  the  shorter,  obovoid  ones  may  contain  seeds,  and  some  even 


48  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

of  these  are  seedless.  Any  of  the  sterile,  fruit-like  bodies  may  be  easily  dis- 
lodged and  on  moist  ground  their  areoles  may  give  rise  to  new  plants.  Such 
a  complete  series  of  more  or  less  fruit-like  structures  might  easily  give  the 
impression  that  these  sterile  propagules  have  arisen  phylogenetically  by  the 
progressive  sterilization  of  the  normal  type  of  fruit,  accompanied  by  an 
increase  in  its  povt^er  of  sprouting  from  its  areoles  until  the  sterile  fruits 
have  become  the  chief  propagative  structures  of  this  species.  The  plausi- 
bility of  this  view  we  shall  consider  in  detail  later  (p.  52).  In  the  mean- 
time, however,  we  must  remember  that  the  so-called  fruit  of  these  opuntias 
is  made  up  largely  of  purely  vegetative  elements,  the  internodes  and  the 
areoles  and  their  products.  It  is  clearly  for  this  reason  that  many  sterile  ova- 
ries, such  as  in  other  angiosperms  (where  they  occur  commonly)  would  soon 
wither  and  fall,  may  in  Opuntia  persist  as  essentially  vegetative  structures. 

Opuntia  catacantha  {0.  rubescens  Salm-Dyck)  is  a  West  Indian  species 
resembling  0.  fulgida  in  certain  respects  more  closely  than  any  other  opuntia 
studied.  It  is  tree-like,  with  very  flat,  paddle-shaped  or  scimitar-shaped 
joints,  which  in  the  variety  studied  are  without  spines.  The  fruits  persist 
from  one  season  to  the  next  and  then  bear  primary  flowers  of  the  latter  sea- 
son. These  may  bear  secondary  flowers  and  the  latter  form  tertiary  ones, 
and  thus  chains  of  fruits  consisting  of  at  least  6  or  8  links  may  finally  be 
formed  (fig.  90).  These  fruits  vary  in  size,  form,  and  internal  structure. 
The  flowers  and  young  fruits  are  slender  and  obconical  or  sometimes  slightly 
bent  (fig.  90).  As  the  fruits  mature  they  increase  considerably  in  size, 
often  to  about  twice  the  original  length  (50  to  60  mm.)  and  to  4  or  5  times 
their  original  thickness.  Mature  fruits  are  often  flattened  until  only  half  as 
thick  on  one  transverse  axis  as  on  the  other  (lY  by  30  mm.  for  example,  in 
one  sho\ATi  in  fig.  9).  Few  of  the  fruits  have  fertile  seeds.  'None  of  those 
dissected  by  the  writer  had  good  seeds,  but  a  few  ripe  seeds  were  sent  him 
by  a  correspondent,  the  Rev.  A.  B.  Romig,  of  St.  Thomas,  Virgin  Islands. 

The  seed  remnants  found  are  of  various  sizes  up  to  about  half-growm  seeds, 
but  all  are  brown  and  withered.  No  definite  information  is  available 
regarding  the  ability  of  these  fruits  to  sprout  to  new  shoots,  but  the  fact  that 
they  are  nearly  always  sterile  suggests  that  they  may  serve  as  propagules  just 
as  the  fruits  of  0.  fulgida  do.  This  possibility  is  rendered  more  plausible  by 
the  fact  that  in  another  spiny  variety  of  Opuntia  catacantha  (0.  monili- 
formis Haworth)  collected  on  Mona  Islands,  near  Haiti,  by  Dr.  N".  L.  Brit- 
ton,  chains  of  short  joints  1.5  by  3'  cm.  long  are  formed,  which  are  said  to 
sprout  to  new  plants.  These  are  spiny  and  except  in  size  are  like  the  regular 
vegetative  joints.  They  are  not  pseudo-fruits  like  those  described  in  0. 
leptocaulis. 

In  PeiresJcia  guamacho  is  found  the  most  striking  case  of  proliferation  of 
the  flower  that  has  been  seen  outside  the  genus  Opuntia,  and  it  appears  the 
more  remarkable  because  of  the  large  bracts  and  long  stalks  of  the  successive 
flowers.     In  this  species,  as  it  grows  in  greenhouses  in  Washington,  each 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  49 

flower  usually  bears  4  bracts  and  each  of  these  has  a  secondary  flower  in  its 
axil.  The  stalks  of  a  secondary  flower  may  get  to  be  a  centimeter  long  or 
more  and  this  gives  the  flower-cluster  quite  a  different  appearance  from  that 
of  an  opuntia,  though  its  general  plan  is  the  same  (figs.  92,  93).  One  or 
two,  rarely  more,  of  the  flowers  in  such  a  group  may  develop  fruits  (cf., 
Delavaud,  1858).  Each  globular,  fleshy  fruit  bears  a  well-defined  areole 
above  each  bract-scar  and  usually  contains  from  one  to  several  large,  flat 
seeds.  These  seeds  germinate  readily,  but  all  attempts  to  induce  the  areoles 
of  a  detached,  unripened  fruit  to  proliferate  to  a  shoot  failed.  No  case  of 
the  vegetative  proliferation  of  the  areole  of  a  fruit  was  observed.  In  a 
similar  species  of  Peireskia  growing  at  Chico,  a  single  parent  fruit  some- 
times bore  5  secondary  fruits  and  the  basal  secondary  ones  not  uncommonly 
bore  in  turn  tertiary  fruits ;  that  is,  three  generations  of  fruits  were  formed 
in  a  single  season. 

In  the  second  series  of  species  mentioned  on  page  47  the  attached  friiit 
sometimes  proliferates  to  form  vegetative  joints  as  well  as  to  give  rise  to 
flower  buds.  All  but  one  of  these  opuntias  resemble  0.  fulgida  in  that  they 
form  these  vegetative  shoots  only  rarely.  Thus,  in  the  flat-jointed  species, 
Opuntia  rufida,  from  Doctor  Rose's  collection  in  Washington,  in  0.  spinosis- 
sima  from  Jamaica,  and  in  0.  discata,  studied  in  the  field  at  Tucson,  the 
development  of  vegetative  shoots  from  attached  fruits  occurred  very  infre- 
quently (fig.  94).  Hildebrandt  (1888,  p.  112)  has  reported  such  a  case  in  a 
flat-jointed  Opuntia  gro\\dng  in  Freiburg.  He  attributed  this  imusual 
behavior  to  exceptionally  good  nutrition  of  the  cultivated  specimens.  Pro- 
liferation of  the  fruits  I  foimd  not  at  all  uncommon  in  a  number  of  the 
above-mentioned  opuntias,  and  of  other  flat  opuntias  growing  in  the  collec- 
tion of  Doctor  Griffiths,  at  Chico,  California  {cf.  Griffiths,  1913).  In  the 
cylindrical  species,  0.  versicolor,  the  occurrence  of  such  a  proliferation  of 
the  attached  fruit  to  vegetative  joints  is  relatively  very  rare,  only  3  or  4  cases 
being  seen  in  hundreds  of  plants  examined.  In  two  at  least  of  the  three 
cases  of  this  tjq^e  observed  the  proliferating  fruit  was  evidently  a  gall  fruit 
(fig.  85 ) .  This  fact  of  itself  proves  that  these  gall  fruits  do  not  always  drop 
off  during  the  second  spring.  It  is  doubtful,  however,  if  they  are  capable, 
except  in  the  rarest  instances,  of  holding  on  long  enough  to  make  their  vegeta- 
tive offshoots  an  important  part  of  the  branch  system.  The  irregularity  in 
branching,  usually  resulting  from  proliferation  of  a  gall  fruit  to  a  branch, 
should  make  this  phenomenon  discoverable  several  years  after  its  occurrence 
(figs.  85,  86).  No  certain  cases  could  be  found,  however,  that  indicated 
clearly  the  persistence  of  such  a  fruit-borne  branch  for  more  than  a  year  or 
two.  This  sort  of  proliferation  is  practically  identical  with  that  occurring 
so  rarely  in  Opuntia  fulgida,  except  that  in  the  latter  the  proliferating  fruit 
is  in  other  res])ects  often  a  normal  one. 

On  the  contrary,  in  the  round-jointed  Opuntia  arhuscula,  the  proliferation 
of  attached  fruits  to  form  vegetative  shoots  is,  under  some  conditions  at  least, 
4 


50  THE   FEUIT   OF   OPUNTIA   FULGIDA. 

not  a  rarity,  but  a  very  common  occurrence.  The  fruit  of  this  species  is  per- 
sistently green,  shows  no  sign  of  ripening,  and  not  more  than  5  per  cent  of 
them  have  well-ripened  seeds.  This  fruit  is  pear-shaped,  rather  slender, 
smooth,  and  spineless,  like  that  of  0.  fulgida,  and  has  rather  prominent 
areoles.  The  areoles  of  the  primary  flower  very  often  proliferate  to  secon- 
dary flowers  and  the  fruits  commonly  persist  over  one  or  more  winters.  Most 
of  the  persistent  fruits  are  single,  but  chains  of  two  are  common  and  chains  of 
3  or  4  links  are  not  infrequent.  The  fruits  of  the  upper  part  of  the  rather 
bush-like  plant  seem  always  to  produce  only  flowers  so  long  as  they  remain 
attached.  Many  of  the  fruits  of  the  lower  branches,  however,  often  give  rise, 
from  one  or  more  up  to  6  or  8  of  the  distal  areoles,  to  rather  slender,  con- 
densed branches,  bearing  numerous  prominent  areoles  (  fig,  95  ) .  These  con- 
densed branches,  which  may  also  arise  on  vegetative  joints,  are  vertical  in 
position,  are  about  4  or  5  mm.  in  diameter,  and  20  to  50  mm.  long.  If  these 
short  branches  are  left  on  the  plant  they  may  thicken  somewhat  and  become 
more  like  the  normal,  terete,  vegetative  shoots.  ISTo  evidence  was  obtained, 
however,  that  the  persistent  fruit  and  its  vegetative  offshoot  ever  become 
incorporated  into  the  permanent  branch  system  of  the  parent  plant.  These 
condensed  branches  apparently  play  no  important  part  in  the  development  of 
this  opuntia,  except  when  the  fruits  or  the  branches  alone  fall  to  the  ground, 
there  to  take  root  and  thus  start  new  plants. 

Wliat  has  been  said  of  the  proliferation  of  the  fruits  in  the  various 
Opuntias  and  in  Peireskia  indicates  that  each  is  peculiar  in  its  own  way  in 
regard  to  the  ripening  of  the  fruit,  in  its  persistence,  and  its  proliferation  to 
flowers  or  to  vegetative  shoots. 

In  the  first  place,  the  fruits  of  two  species,  0.  arhuscula  and  0.  catacantha, 
have  fruits  resembling  those  of  0.  fulgida  in  that  they  normally  fail 
to  ripen.  The  other  four  species,  0.  cylindrica,  0.  leptocaulis,  0.  rufida, 
and  0.  versicolor,  apparently  fail  to  ripen  only  because  of  some  unusual 
condition  within  or  about  them. 

Secondly,  of  the  .eight  species  of  Opuntia  just  mentioned  only  two,  0. 
arhuscula  and  0.  catacantha,  seem  to  resemble  0.  fidgida  in  having  normally 
persistent  fruits.  In  the  others  this  is  the  less  usual  thing,  due  in  all  cases 
perhaps,  as  it  evidently  is  in  0.  versicolor,  to  some  unusual  cause,  such  as  the 
stimulus  caused  by  Asphondylia.  There  are  many  other  species  also  in 
which,  as  was  noted  by  Griffiths  (1913),  the  abnormal  conditions  of  growth 
provided  in  cultivation  frequently  induce  persistence  of  the  fruits. 

Thirdly,  the  proliferation  of  the  areoles  of  the  ovary  to  flowers,  while 
apparently  a  nonnal  occurrence  in  0.  arhuscula,  0.  catacantha,  and  the 
closely  related  0.  spinosissima,  as  it  also  is  in  0.  fulgida  and  Peireskia,  is 
a  rarer  phenomenon  in  the  other  four  species  mentioned,  and  in  them  occurs, 
usually,  if  not  always,  under  abnormal  conditions. 

Finally,  the  proliferation  of  the  areoles  of  fallen  fruits  to  roots  and  shoots, 
thus  to  form  new  plants,  may  apparently  occur  in  any  of  the  seven  opuntias, 
but  not  in  the  soft,  quickly  ripening  fruits  of  Peireshia. 


THE   FRUIT   OF   OFUNTIA   FULGIDA.  51 

Taking  into  account  all  four  of  the  peculiarities  mentioned,  it  is  clear  that 
Opimtia  fulgida  is,  on  the  whole,  the  most  generally  peculiar  type  studied, 
followed  most  closely  perhaps  by  0.  arhuscula  and  0.  catacantha.  At  the 
other  end  of  the  graded  series  is  Opuntia  versicolor,  which  behaves  like  a 
normal  angiosperm  in  most,  or  perhaps  all,  cases  where  it  is  not  stimulated 
by  the  cactus  fly,  Aspliondylia. 

It  is  evident  (from  observations  made  in  Arizona,  in  the  cactus  garden 
of  the  Bureau  of  Plant  Industry  established  by  Doctor  Griffiths  at  Chico, 
and  from  records  in  the  literature,  of  cases  such  as  Opuntia  prolifera, 
0.  cJiolla,  0.  spinodor,  etc.)  that  many  other  species  of  Opuntia  fit  in  at 
various  points  in  the  series  described  above.  In  other  words,  in  the  genus 
Opuntia  the  ovary,  in  flower  and  in  fruit,  may  assume  now  more,  now  fewer 
of  the  various  functions  of  the  vegetative  joint.  Of  the  Opuntia  fruits  thus 
far  studied  from  this  point  of  view,  that  of  Opuntia  fulgida  seems  not  only 
the  most  atypical  angiospemious  fruit  among  these  Cactacea?,  but  is  perhaps 
also  the  most  aberrant  (shoot-like)  fruit  to  be  found  in  all  angiosperms. 

PROLIFERATION   OF   JOINTS   AND   FRUITS   IN 
RELATION  TO  THE  STERILITY  OF  FRUITS. 

From  what  has  been  said  above  of  Opuntia  fulgida  it  is  evident  that  the 
propagation  of  this  cactus  has  ceased  to  depend  chiefly  on  the  development  of 
fertile  seeds,  and  so  of  seedlings,  but  is  accomplished  more  largely  by  the 
vegetatix-e  sprouting  of  fallen  joints  and  of  fallen  fruits.  The  propagation 
of  the  species  by  the  rooting  of  joints  or  parts  of  joints  is  rather  common  in 
several  genera  of  Cactacea?,  such  as  Cereus,  Mammillaria  (Goebel,  1889), 
Peircslda,  etc.,  in  addition  to  many  and  probably  most  species  of  Opuntia. 
The  propagation  by  means  of  the  fallen  fniit  is  probably  common  among 
opuntias  with  persistent  fruits.  Besides  its  occurrence  in  the  forms  already 
mentioned,  it  is  apparently  common  also  in  0.  prolifera,  0.  cholla,  0. 
spinosior,  etc. 

In  a  number  of  other  opuntias  and  certain  other  genera  there  are  devel- 
oped, in  addition  to  the  true  fruits  and  ordinary  vegetative  joints,  more 
specialized  short  joints  which  are  often  bead-like  or  more  or  less  fruit-like  in 
character.  These  occur,  for  example,  in  0.  arhuscula,  0.  catacantha, 
0.  fulgida,  0.  leptocaulis,  0.  tetracantha  (Toimiey,  1905),  Mammillaria 
gracilis  (Goebel,  1889),  a  species  of  Cereus,  etc.  In  each  case  these  struc- 
tures are  capable,  after  falling,  of  taking  root  in  the  soil  and  tlius  starting 
new  plants. 

Taking  this  whole  series  of  structures,  together  with  the  various  sorts  of 
joint  fruits  that  occur  in  0.  fuJgida  and  many  other  opuntias,  and  the  many 
seedless  opuntia  fruits,  it  might  be  assumed  that  the  fruit  of  these  Cactacefe, 
and  especially  of  the  opuntias,  is  losing  its  primary  function  of  seed-produc- 
tion. ^  It  is  evident  at  least  that  the  production  of  new  plantlets  and  the 
function  of  dissemination  has  been  taken  over  in  large  part  by  these  various 


52  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

vegetative  propagating  bodies.  This  raises  the  question  whether  the  seed- 
bearing  fruit  of  all  opuntias  is  on  the  way  to  extinction,  through  a  progres- 
sive loss  of  the  seed-bearing  function,  which  may  be  expected  to  end  in  the 
evolution  of  a  totally  sterile  fruit,  having  no  propagative  functions  other 
than  those  that  can  be  equally  performed  by  vegetative  joints. 

It  is  true  that  the  vegetative  joints  and  both  the  fertile  and  sterile  fruits 
resemble  each  other  greatly  in  their  capacity  for  proliferation.  There  seems 
no  adequate  reason,  however,  for  assuming  that  either  the  proliferating  habit 
or  the  fundamental  structure  of  the  fruit  is  a  secondary  thing  in  the  evolution 
of  the  opuntias  (Toumey,  1895).  On  the  contrary,  it  is  natural  that  the 
thick-skinned,  water-stored  joints  of  these  cacti  should  prove  capable  of  per- 
sisting on  moderately  moist  soil  until  rooted  deeply  enough  to  secure  a  water- 
supply  adequate  for  the  starting  of  a  young  plant.  The  fruit  being,  as  we 
have  seen,  really  a  stem  in  organization,  up  to  the  latest  phase  of  its  develop- 
ment, it  is  also  very  naturally  capable  of  proliferatiou  to  root  and  shoot. 
The  capacity  of  joint  and  fruit  for  persistence  and  proliferation  is  probably 
as  old  as  the  fleshy  character  of  the  family.  The  persistence  of  the  sterile 
fruits,  at  least  to  maturity,  is  not  a  really  surprising  thing,  in  view  of  the 
preponderatingly  vegetative  and  stem-like  character  of  the  bulk  of  the  wall 
of  the  ovary.  Sterile  ovaries  occur  in  many  species  of  angiosperms,  but  in 
most  of  these  the  carpels  constitute  the  bulk  of  the  fruit.  Therefore,  when 
the  seeds  are  wanting  in  these  forms,  and  the  carpels  as  usual  fail  to  develop, 
no  fruit  is  formed  and  the  flower  bud  soon  withers  and  drops  off.  In 
Opuntia,  on  the  contrary,  even  if  the  seeds  and  carpellary  portion  of  the 
fruit  do  fail  to  develop,  the  basal  stem-like  part  may  go  on,  practically 
unhindered  in  its  vegetative  growth,  and  mature  quite  normally. 

When  these  facts  concerning  the  comparative  structure  and  behavior  of  the 
stem  and  ovary  of  Opuntia  are  considered,  in  conjunction  with  the  fact  that 
leaves  are  present  on  both  and  with  the  undoubted  similarity  of  Opuntia  to 
Peireskia,  there  seems  no  adequate  reason  for  believing  that  the  fniit  of 
Opuntia  is,  structurally,  an  advanced  type  among  the  Cactacese.  On  the 
contrary,  Opuntia  and  its  close  relative  Peireskia  (which  also  has  a  leafy 
ovary,  the  areoles  of  which,  in  the  flower,  proliferate  to  secondary  flowers) 
probably  show  us  the  simplest  type  of  the  inferior  (submerged)  ovary  char- 
acteristic of  the  Cactaceae.  The  capacity  of  the  Opuntia  fruit  for  persistence 
and  proliferation  is  to  be  regarded  as  the  natural  outcome  of  its  original 
morphological  composition — i.  e.,  a  joint  with  an  ovary  immersed  in  its  apex. 
It  seems  clearly  not  a  result  of  a  marked  degeneration  of  a  once  less  stem- 
like and  more  constantly  fertile  fruit,  such  as  has  been  assumed  ])y  some 
investigators  to  have  been  present  during  the  evolution  of  the  genus.  The 
more  specialized  or  highly  evolved  flowers  and  fruits,  structurally,  among  the 
Cactacese  are  probably  to  be  sought  in  such  genera  as  Cereus,  Rhipsalis,  and 
Mammillaria. 


THE   FEUIT   OF   OPUNTIA   FULGIDA.  53 

SUMMARY  AND  CONCLUSIONS. 

The  fruits  of  certain  opuntias  differ  from  those  of  other  angiosperms, 
except  those  of  some  Australian  "  bottle-bnish  "  trees,  in  not  ripening  and 
then  either  opening  or  falling  from  the  plant  when  the  seeds  are  mature. 
On  the  contrary,  the  peculiar  fruits  of  these  Cactacese  and  MyrtacesG  remain 
attached  to  the  plant  and  actively  growing  for  several  or  many  years. 

The  persistent  fruit  of  Opuntia  fulgida  is  still  more  abnormal  in  another 
respect,  for  it  not  only  remains  attached,  unripened  and  steadily  growing, 
season  after  season,  but  the  seeds  are  never  shed  from  the  fruit.  Further- 
more, the  matured  fruit  itself,  or  even  the  ovary  of  the  unopened  flower, 
while  still  attached  to  the  tree,  may  give  rise  to  secondary  flowers  and  so  to 
other  fruits.  Four  or  five  generations  of  flowers  and  fruits  may  thus  be 
formed  in  a  single  season.  Finally,  if  a  mature  fruit  falls  on  moist  soil  it 
may  develop  adventitious  roots  and  shoots  and  thus  initiate  a  new  plant. 

This  tree-like  opuntia  has  a  tuberculate  and  spiny  cylindrical  stem  and 
branches,  the  fleshy  joints  of  which  on  separation  readily  sprout  to  new 
plants. 

The  early  development  of  the  ovary  of  the  flower  in  Opuntia  fulgida 
closely  resembles  that  of  a  young  vegetative  joint,  and  the  structure  resulting 
from  this  early  development,  with  its  minute,  evanescent  leaves,  its  tubercles, 
and  axillary  areoles,  is  entirely  stem-like  in  appearance.  Only  with  the 
initiation  of  the  perianth,  stamens,  and  carpels  does  the  fertile  joint  become 
distinctly  flower-like  in  character.  The  ovarian  cavity  finally  becomes  com- 
pletely buried  in  the  stem-like,  basal  portion  of  the  ovary,  by  the  more  rapid 
growth  of  this  portion  upward  and  inward  about  the  base  of  the  carpels. 
The  whole  outer  wall  of  the  ovary  and  fruit  is  thus  a  stem  in  its  morpho- 
logical origin.  This  is  clearly  indicated  not  only  by  the  more  general  fea- 
tures of  development  noted,  but  also  by  the  identity  in  details  of  development 
and  structure  of  the  tubercles  and  areoles,  and  of  the  photosynthetic  and 
water-storing  tissues  in  the  stem  and  fruit.  In  its  physiological  capacity 
for  persistence  and  for  proliferation  to  flowers  and  shoots,  the  wall  of  the 
fruit  shows  again  its  essential  identity  with  the  stem.  Finally,  the  graded 
series  of  structures  intermediate  in  character  between  joint  and  fruit,  sen-e 
to  further  emphasize  the  likeness  of  the  two. 

The  development  of  the  flower  of  this  opuntia  indicates  that  it  has  evolved 
from  one  with  an  originally  superior  ovary  through  the  progressive  submer- 
gence of  this  ovary  by  the  more  vigorous  growth  of  parts  of  the  fertile  joint 
that  were  laid  down  before  the  carpels  themselves  were  even  initiated.  Tliis 
is  probably  a  relatively  primitive  type  of  flower  among  the  Cactacese,  from 
which  the  type  found  in  Cereus  and  Echinocactus  has  been  derived. 

The  perianth,  the  stamens,  and  the  style  are  cut  off  from  the  top  of  the 
ovary,  a  day  or  two  after  the  flower  withers,  by  the  formation  of  a  highly 
developed,  cup-shaped  abscission  layer.     Any  cells  of  the  fruit  except  those 


54  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

of  the  vascular  bundle  may  participate  in  the  formation  of  the  cells  of  the 
abscission  layer.  The  cells  of  the  vascular  bundle  in  line  with  the  abscission 
layer  seem  to  degenerate  and  rupture  as  a  result  of  the  split  in  the  adjoining 
tissues.  The  whole  funnel-like  scar  left  at  the  top  of  the  ovary  by  the  fall  of 
the  perianth  is  soon  protected  by  several  layers  of  periderm.  These  arise 
from  a  phellogen  formed  but  a  few  layers  within  the  abscission  layer.  One, 
or  sometimes  several,  layers  of  this  periderm  may  have  the  cell-walls  greatly 
thickened  to  form  a  schlerenchymatous  protective  layer. 

From  the  axillary  buds,  or  areoles,  of  the  primary  flowers  that  open  in 
May,  arise  secondary  flowers  which  open  in  June.  From  areoles  of  these,  in 
turn,  tertiary  flowers  open  in  July,  and  on  the  latter  quaternary  flowers 
bloom  forth  in  August.  Thus  four  and  sometimes  five  generations  of  flowers 
may  be  formed  each  season.  Often  two  or  three  and  sometimes  four  genera- 
tions of  persistent  fruits  may  thus  arise  in  a  single  summer. 

The  number  of  well-matured  seeds  occurring  in  a  fruit  may  range  from  0 
to  100  or  even  200.  Large  numbers  of  sterile  seed-rudiments  of  various 
sizes  are  found  in  most  fruits,  some  of  them  evidently  having  degenerated 
soon  after  their  initiation.  The  fertile  seed  contains  a  large  coiled  embryo 
and  a  small  mass  of  endosperm  in  the  loop  between  radicle  and  cotyledons. 
The  seeds  of  this  opuntia  have  not,  so  far  as  is  recorded,  been  Icnown  to  ger- 
minate in  the  field  under  natural  conditions.  They  were  germinated  in  the 
laboratory  by  slightly  chipping  the  seed-coat.  The  seeds  may  remain 
unchanged  and  capable  of  germination  for  several  or  many  years  while 
embedded  in  the  pulp  of  the  persistent,  attached  fruits,  or  even  in  that  of 
fallen,  rooted  ones.  The  seeds  are  set  free  in  nature  only  by  the  decay  of 
the  pulp  of  the  fallen  fruit,  or  when  the  fruits  are  eaten  by  browsing  animals. 
It  is  possible  that  the  failure  of  the  seeds  to  germinate  in  the  moist  pulp  of 
the  fruit,  even  during  the  hot  summer,  may  be  due  to  the  impenetrability  of 
the  seed-coat.  Even  chipped  seeds,  however,  do  not  germinate  in  this 
medium  as  they  do  in  soil  or  on  wet  filter-paper,  which  suggests  that  the  pulp 
may  have  an  inhibitory  effect  on  some  process  connected  with  germination. 

The  mature  fruit,  unripened,  remains  attached  to  the  tree  after  the  ripen- 
ing of  the  seeds.  It  may  thus  persist  and  grow  year  after  year,  by  the  aid 
of  a  cambium  ring.  It  is  this  persistent  fruit  that  gives  rise  to  flowers,  just 
as  a  stem  would.  By  the  proliferation  of  the  primary  flowers,  so  formed, 
secondary  and  tertiary  flowers  arise  and  develop  to  persistent  fruits,  two, 
three,  or  even  four  generations  per  season.  In  the  course  of  several  years  a 
cluster  may  arise  containing  scores  of  fruits  with  sometimes  10,  12,  or  even 
14  generations  of  fruits  in  a  continuous  linear  series. 

Only  in  two  cases,  among  hundreds  examined,  were  attached  fruits  found 
proliferating  to  vegetative  joints. 

Fallen  fruits  that  rest  on  moist  ground  may  give  rise  to  adventitious  roots, 
from  areoles,  perianth-scar,  or  stalk-scar,  and  then  to  vegetative  shoots,  from 
areoles  only,  and  thus  initiate  new  individual  plants.     In  nature  this  origin 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  55 

of  new  plants  from  fallen  fruits  is  the  most  important  means,  next  to  prolif- 
eration of  fallen  joints,  of  the  multiplication  and  dispersal  of  this  cactus. 

The  difference  between  the  product  of  an  areole  of  an  attached  fruit  and 
one  of  a  fallen  fruit  that  has  rooted  seems  probably  due  to  a  difference  in  the 
kind  of  nutritive  (organ-building)  material  brought  to  the  two  areoles  under 
these  different  conditions.  It  is  possible  that  the  presence  of  roots  on  the 
fallen  fruit  inhibits  the  formation  of  flowers  by  it. 

Persistence  and  proliferation  of  the  fruit,  though  not  elsewhere  as  fre- 
quent as  in  Opuntm  fulgida,  is  not  unknoA\Ti  in  other  species.  In  Opuntia 
versicolor,  as  also  in  several  flat-jointed  opuntias,  the  frequent  persistence  of 
the  fruit  or  even  of  the  unopened  flower  is  the  result  of  the  puncture  of  the 
flower-bud  by  the  cactus  fly,  which  lays  its  eggs  in  it.  In  other  cases,  like  0. 
catacantha,  the  factors  that  inhibit  ripening  and  induce  persistence  are  as 
undetermined  as  they  are  in  the  case  of  0.  fulgida. 

The  fact  that  Opuntia  fulgida  and  other  species  have  series  of  fruits  show- 
ing various  degrees  of  sterility,  from  those  with  scores  of  seeds  to  those  that 
are  entirely  seedless,  can  not  be  taken  as  conclusive  evidence  that  seed-produc- 
tion is  really  on  the  way  to  complete  extinction  in  these  plants.  ISTeither  is 
the  corollary  that  propagation  by  seeds  is  being  replaced  by  the  proliferation 
to  new  plants  of  fallen  fruits  as  significant  as  it  might  at  first  seem.  On  the 
contrary,  the  stem-like  character  of  the  fruits  in  this  genus  results  in  the  per- 
sistence of  many  sterile  ovaries,  such  as  would,  in  many  less  fleshy  angio- 
sperms,  wither  and  fall  off  soon  after  blooming,  instead  of  maturing  into 
seedless  fruits,  as  they  do  here. 


56  THE  FRUIT   OF   OPUNTIA  FULGIDA. 

LITERATURE  CITED. 

Caspaei,  H.     1883.     Beitrage  der  Kenntniss  des  Hautgewebes  der  Cacteen.     Zeit. 

fur  Naturwiss.,  2,  pp.  30-80. 
Cbockeb,  W.     1906.     Role  of  Seed-coats  in  Delayed  Germination.    Bot.  Gaz.,  42,  pp. 

265-290. 
Daebishire,  O.  V.     1904.     Observations  on  Mammillaria  elongata.    Ann.  of  Bot.,  18, 

pp.  375-416. 
Debet,  M.  H.     1846.     Proliferation  d.  Fruchtknotens  lieferte  eine  Opuntia  Salm- 

Dyckiana.    Verhandl.  d.  Naturhist.  Ver.  Preuss.  Rheinl.,  3,  83,  84. 
Delavaud,  M.  C.     1858.     Inflorescence  du  Peireshia  bleo.    Bull.  Soc.  Bot.  de  France, 

5,  p.  685. 
Delbbouck,  C.     1875.     Die  Pfianzenstacheln.     Hanstein  Botan.,  Abhandl.  2,  pp.  1-119. 
Engelmann,  W.     1887.    "  Cactacese  of  the  Boundary,"  in  Botanical  Works.     Edited 

by  W.  Trelease  and  Asa  Gray.    Cambridge,    p.  212. 
EwAET,  A.  J.    1907.    The  Delayed  Dehiscence  of  CalUstemon  rigida.    Ann.  of  Bot,  21, 

pp.  125-137. 
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.     1898.     The  Comparative  Morphology  of  the  Embryos  and  Seedlings  of  the 

Cactace*.    Ann.  of  Bot.,  12,  pp.  423-474. 
GOEBEL,   Karl.     1886.     Zur   Entwickelungsgeschichte   des   Unterstandigen   Frucht- 
knotens.   Bot.  Zeit,  44,  pp.  729-738. 

.     1889.    Pflanzenbiologischeschilderungen.    I.  Kakteen,  pp.  67-108. 

Griffiths,  D.     1913.     Behavior,  Under   Cultural   Conditions,   of   Species   of  Cacti 

Known  as  Opuntia.    Bull.  No.  31,  U.  S.  Depart,  of  Agricult. 
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des  Naturalistes  de  Moscow,  32,  part  II,  pp.  585-603. 
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.     1916.    Abscission  in  MiraUlis  jalapa.    Bot  Gaz.,  61,  pp.  213-230. 

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pp.  599-742. 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  57 

EXPLANATION  OF  PLATES. 

ABBREVIATIONS  USED  IN  PLATES. 

a.,  areole  or  cushion  formed  by  axillary  bud;  a.  I.,  abscission  layer;  &.,  bristle  or 
glochidium ;  bb.,  barb  of  bristle  or  of  spine;  c,  carpel;  ck.,  cork;  cm.,  cambium; 
c.  c,  crystal  containing  cell;  c.  t.,  conducting  tissue  of  style;  c.  to.,  cell-wall;  c,  epi- 
dermis; em.,  embryo;  ep.,  endosperm;  /.,  flower;  fu.,  funiculus  or  stalk  of  ovule; 
g.  c,  guard-cell;  g.  p.,  growing-point;  h.,  hypodermis;  i.,  integument;  I.,  leaf;  l.  s.,  leaf- 
scar;  n.,  nectary;  o.,  ovary;  ov.,  ovule;  p.,  petal;  ph.,  phellogen;  pi.,  palisade;  p.  s., 
perianth-scar;  r.,  root;  s.,  sepal;  sa.,  stamen;  s.  c,  slime-cell;  sd.,  seed;  sg.,  stigma; 
sh.,  sheath  of  spine;  so.,  stoma;  sp.,  spine;  st.,  stem;  sy.,  style;  t.,  trichome;  tb., 
tubercle;  v.  b.,  vascular  bundle;  w.  f.  wall  of  fruit. 

Frontispiece. 
Photograph  of  a  mature  plant  of  Opuntia  fulgida  on  reservation  of  Desert  Laboratory 
at  Tucson,  showing  a  frequent  type  of  forked  trunk,  due  to  injury  of  main 
axis,  also  the  branching  habit  and  clusters  of  fruit.  The  nesting  bird  is  the 
cactus  wren,  Heleodytes  brunneicapillus  couesi  (Sharpe).  [D.  T.  MacDougal 
photo.] 

Plate  1.    Photographs  of  Opuntia  fulgida. 

Fig.  1.  A  fruiting  plant  growing  in  the  desert  at  Tucson.  Photographed  April  25, 
1915.     X  0.03. 

Fig.  2.  Heavily  fruiting  branches  of  a  tree  on  the  campus  of  the  University  of  Arizona. 
The  largest  cluster  included  more  than  100  fruits.  Photographed  in 
May  1912.     X  0.1. 

Fig.  3.  A  single  large  cluster  of  fruits  from  the  same  tree  as  figure  2.     X  0.3. 

Fig.  4.  A  vegetative  joint  and  fruits  of  1914  bearing  buds  of  flowers  of  1915.  Photo- 
graphed in  May  1915.     X  0.6. 

Plate  2.    Photographs  of  0.  fulgida. 

Fig.  5.  Tip  of  a  vegetative  joint  with  young  joints  still  bearing  the  evanescent  leaves, 
showing  also  areoles  with  spines  and  nectaries.     X  0.9. 

Fig.  6.  Four  generations  of  flowers  and  fruits  developed  from  a  vegetative  joint  in  the 
season  of  1915.  This  cluster,  collected  at  Tucson  and  photographed  in 
mid-July  1915,  shows  the  relative  sizes  of  the  four  generations.  No.  I 
opened  in  May;  II  in  June;  III,  if  not  removed  from  the  plant,  would 
have  opened  in  late  July;  IV  in  late  August.  Note  that  some  members 
of  generation  III  (at  right  below)  have  but  barely  pushed  out  of  the 
areole.     X  0.6. 

Fig.  7  a,  b,  c.  Joint-fruits  and  pseudo-fruits,  showing  several  structures  combining  in 
various  degrees  the  characters  of  vegetative  joint  and  fruit.     X  0.45. 

Fig.  8.  Cluster  of  11  secondary  fruits  borne  on  a  single  primary  fruit  showing  the 
persistence  of  fruits  over  one,  two,  or  more  winters.  Photographed 
April  1915.    X  1. 

Fig.  9a.  Joint  of  1914  bearing  opened  and  withered  flower  of  1915,  the  latter  with 
buds  of  secondary  flowers  on  its  sides.     X  0.6. 

FiQ.  9&.  Similar  joint  bearing  primary  flowers.  In  the  areoles  of  these  are  borne 
trichomes,  nectaries,  and  buds  of  secondary  flowers.  Many  of  the  latter 
bear  numbers  of  the  awl-shaped  deciduous  leaves.     X  0.6. 

Plate  3.     Drawings  of  O.  fulgida. 

Fig.  10.  Radial  section  of  an  axillary  bud  of  flower,  showing  growing-point,  nectary, 

spine,  etc.     X  25. 
Fig.  11.  Longitudinal  section  of  a  flower  bud  through  a  placenta,  mammillae,  leaves, 

two  axillary  buds,  a  nectary,  etc.    X  3.33. 


58  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

Fig.  12.  Enlarged  drawing  of  the  areola  shown  at  left  in  figure  11.  It  would  probably 
have  developed  a  flower  if  it  had  been  left  on  the  plant.    X  25. 

Fig.  1.3.  Part  of  a  longitudinal  and  radial  section  of  an  unopened  flower,  bearing  the 
very  young  bud  of  a  secondary  flower  at  right.    X  5. 

Fig.  14.  Longitudinal  section  of  a  young  flower  bud  arising  from  the  edge  of  the 
perianth-scar  on  the  fruit  of  the  preceding  season,  showing  leaves,  very 
prominent  tubercles,  and  growing-points  of  the  areoles  on  the  adaxial 
faces  of  the  latter;  showing  also  the  shriveled  nectaries  developed  in  the 
preceding  season.     X  12. 

Fig.  15.  Longitudinal  section  of  a  slightly  older  flower  than  that  shown  in  figure  14, 
showing  the  depression  of  the  growing-point  and  the  initiation  of  the 
stamens.     X  18. 

Fig.  16.  Longitudinal  section  of  a  still  more  advanced  flower,  showing  the  much 
sunken  growing-point  and  three  series  of  stamens  on  each  side  of  it. 
X  18. 

Fig.  17.  Longitudinal  section  of  flower  with  all  stamens  and  carpels  initiated.  The 
two  upper  areoles  now  face  upward  instead  of  axially.    X  9. 

Fig.  18.  Part  of  a  longitudinal  section  of  a  young  flower  showing  further  closing  in  of 
the  carpels  above  the  cavity  of  ovary  to  form  stylar  canal.    X  24. 

Fig.  19.  Longitudinal  section  of  upper  part  of  a  slightly  older  flower,  showing  style 
with  free  tips  that  are  to  form  stigmas.     X  17. 

Fig.  20.  A  section,  similar  to  that  in  figure  19,  of  a  flower  in  which  the  placentae  are 
just  distinguishable;  at  base  of  nectary  at  left  is  a  leaf-scar,  and  below 
the  nectary  is  the  vascular  bundle  that  led  to  leaf.    X  8. 

Fig.  21.  Longitudinal  section  of  a  half-matured  flower,  showing  ovules  just  initiated, 
stamens  differentiated  to  anther  and  filament,  etc.     X  5. 

Fig.  22.  Longitudinal  section  of  a  flower  nearly  ready  to  open,  showing  ovules,  style 
with  its  conducting  tissue,  the  papillose  stigmas,  and  tightly  over- 
lapped sepals  and  petals.     X  4.5. 

Plate  4.    Drawings  of  0.  fulgida. 

Fig.  23.  Longitudinal  section  of  a  flower  that  has  just  commenced  to  open,  showing 

ovules,  papillose  recurved  stigma-lobes,  etc.    X  4. 
Fig.  24.  Longitudinal  section  of  a  young  fruit  from  which  the  perianth  and  stamens 

have  recently  fallen,  showing  the  funnel-shaped  scar  with  its  corky 

lining  layer.     X  3. 
Fig.  25.  Longitudinal  section  of  a  primary  fruit  with  ripe  seeds  bearing  a  secondary 

fruit,  showing  relative  size  of  fruits  and  degree  of  development  of  seeds, 

the  connection  of  vascular  systems,  etc.    X  1.5. 
Fig.  26.  Longitudinal  section  of  two  matured  fruits,  one  or  two  years  old,  with 

aborted  seeds  of  various  sizes,  though  fruits  are  plump  and  normal  in 

external  form.     X  1.25. 
Fig.  27.  Longitudinal  section  of  a  mature  fruit,  showing  the  usual  shape  of  fruit,  its 

perianth-scar,  areoles,  and  vascular  system,  also  ripe  seeds,  together  with 

other  seeds  that  have  withered  at  various  stages  of  development.    X  1.25. 
Fig.  28.  Longitudinal  section  of  a  combination  joint  fruit,  one  or  two  years  old, 

showing  the  relatively  small   portion  of  its  length  occupied  by   the 

ovarian  cavity,  which  in  this  case  contained  only  half-matured  withered 

seeds;  showing  also  the  vascular  system  and  the  prominent  tubercles, 

the  one  at  the  right  with  two  spines.     X  1.5. 
Fig.  29.  Transverse  section  of  young  flower  bud  showing  petals,  sepals,  leaves,  and  at 

left  one  spine.    X  10. 
Fig.  30.  Transverse  section  of  same  bud  lower  down,  showing  the  six  stigmas,  petals, 

sepals,  leaves,  and  the  very  prominent  tubercles  with  their  areoles; 

showing  nectaries,  spicules,  etc.     X  5. 
Fig.  31.  Transverse  section  of  the  bud  shown  in  figure  29,  at  level  of  the  styles  and 

stamens.    X  5. 
Fig.  32.  Transverse  section  of  the  bud  shown  in  figure  29,  at  level  of  the  ovarian 

cavity,  showing  first  rudiments  of  ovules.    X  5. 


THE   FRUIT   OF   OPUNTIA   FULGIDA.  59 

Fig.  33.  Transverse  section  of  the  sterile  base  of  the  same  flower  bud  as  that  shown 

in  figure  29,  showing  the  ring  of  vascular  bundles,  with  fascicular 

cambium  already  established.     X  5. 
Fig.  34.  Transverse  section  of  an  older  flower  bud  through  seven  styles  or  stigmas, 

the  stamens,  petals,  and  sepals.     X  5. 
Fig.  35.  Part  of  an  approximately  transverse  section  of  the  flower  shown  in  figure  34, 

giving  details  of  structure  of  style  and  stigma,  including  the  papillose 

lining  of  the  stylar  canal.    X  20. 

Plate  5. 

Fig.  36.  Part  of  transverse  section  of  a  stigma,  showing  papillose  surface,  and  the 
conducting  tissue,  vascular  bundle,  slime-cells,  etc.,  within.     X  75. 

Fig.  37.  Transverse  section  through  the  flower  shown  in  figure  34,  showing  style, 
stamens,  and  wall  of  ovary  with  its  tubercles,  areoles,  etc.    X  3. 

Fig.  38.  Part  of  transverse  section  of  young  ovary,  showing  long-stalked  ovules  with 
integuments  initiated,  and  archesporial  cell  differentiated.     X  24. 

Fig.  39.  Transverse  section  of  a  young  ovary,  showing  seven  placentas  with  numerous 
anatropous  ovules,  also  the  vascular  structure  and  radial  arrangement 
of  photosynthetic  cells  in  tubercle.     X  4. 

Fig.  40.  Transverse  section  of  slightly  older  ovary,  showing  the  filling  up  of  the 
ovarian  cavity  by  the  growth  of  the  young  ovules  and  their  stalks.  Note 
that  the  magnification  is  but  half  that  of  figure  39.     X  2. 

Fig.  41.  Transverse  section  of  mature  ovary  showing  two  nearly  ripe  seeds  and  a 
larger  number  of  sterile  seeds  that  have  ceased  growing  at  various 
stages  of  development.     X  2. 

Fig.  42.  Transverse  section  of  a  persistent,  fertile  fruit  several  years  old,  showing 
seeds  and  their  stalks  completely  filling  ovarian  cavity,  the  great  radial 
growth  of  the  vascular  bundles,  and  the  loss  of  prominence  of  the 
tubercles.     X  2. 

Fig.  43.  Transverse  section  of  the  sterile  base  of  a  fruit  like  that  of  figure  42,  show- 
ing the  large  central  mass  of  water-filled  parenchyma,  the  radial  growth 
of  the  vascular  bundles  from  the  activity  of  the  fascicular  cambium,  and 
the  generally  smooth,  rounded  outline  of  the  surface.    X  2. 

Fig.  44.  Transverse  sections  of  vascular  bundles  from  ovaries  of  various  ages,  all  at 
same  magnification,  to  show  relative  growth  of  the  phloem  and  xylem 
regions  of  the  bundle,  (a)  From  ovary  of  a  flower  from  which  the 
perianth  has  just  fallen;  (ft)  from  one-year  fruit;  (c)  from  a  fruit  6  or 
8  years  old,  35  millimeters  in  diameter.     X  11. 

Fig.  45.  Transverse  section  of  lower  third  of  nearly  mature  leaf  from  the  ovary  of  a 
flower  about  ready  to  open,  showing  flattened  form,  vascular  system, 
slime-cells,  and  the  slightly  specialized  palisade.   X  42. 

Fig.  46.  Transverse  sections  near  tip  of  leaf  from  flower  bud,  showing  small  vascular 
strand,  slightly  developed  palisade,  and  large  air-canals.     X  50. 

Fig.  47.  Surface  view  of  a  flower  bud  some  time  before  opening,  showing  sepals  and 
petals  and  six  areoles.  Two  of  the  latter  have  already  initiated  flower 
buds,  in  which  the  spindle-shaped  leaves  of  lower  part  of  ovary  and  the 
flattened  sepals  and  petals  can  be  distinguished.  Each  of  the  remaining 
areoles  shows  a  leaf-scar  and  the  dense  tuft  of  trichomes,  with  its  inner 
border  of  spicules  and  its  one  or  several  embedded  nectaries.     X  3. 

Plate  6. 

Fig.  48.  Part  of  transverse  section  near  top  of  opening  flower,  showing  upper  surface 
of  an  areole  and  a  cross-section  of  the  subtending  leaf.  Among  the 
trichomes  of  the  areole  are  ten  nectaries,  and  about  its  inner  border  is  a 
group  of  a  dozen  or  more  bristles  or  glochidia.     X  2. 

Fig.  49.  Part  of  a  transverse  section  of  a  recently  opened  primary  flower,  showing 
edge  of  cup  of  flower  and  three  tubercles,  one  of  them  bearing  an  areole 
with  two  nectaries  and  the  bud  of  a  secondary  flower.  The  arrow  indi- 
cates the  sagittal  plane.    X  3. 


60  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

Fig.  50.  Radial  or  sagittal  section  of  an  areole  of  a  primary  flower,  showing  the 
growing-point  or  stem  apex  of  the  areole,  its  bordering  groups  of  bristles 
and  trichomes,  two  nectaries,  and  the  subtending  leaf.     X  18. 

Fig.  51.  Three  typical  trichomes  from  an  areole,  showing  their  swollen  tops  and 
slender  bases.    X  45. 

Fig.  52.  A  single  trichome  from  an  areole,  showing  thick-walled  pitted  cells  of  tip. 
X  110. 

Fig.  53.  A  mature  spine,  showing  the  swelling  at  base,  the  barbed  tip,  and  the  trans- 
parent sheath  attached  for  its  lower  sixth,  while  the  upper  five-sixths  of 
it  is  loose  and  has  slipped  down  by  splitting  and  folding  near  the  base 
to  leave  the  tip  of  the  spine  exposed.     X  5. 

Fig.  54.  The  upper  fourth  of  an  immature  spine,  showing  in  detail  the  intact  striated 
sheath  and  the  barbed  tip  of  the  spine  itself.    X  50. 

Fig.  55.  Transverse  section  of  a  spine  from  a  young  fruit,  showing  the  sheath  of 
loosely  packed  hairs  and  the  spine  with  its  core  of  closely  compacted 
small  cells  and  its  outer  layers  of  large  thick-walled  barb-cells.     X  110. 

Fig.  56.  Part  of  a  sagittal  section  of  an  areole,  showing  a  nectary  with  bristles  and 
trichomes  at  its  base,  with  the  separated,  cutinized  layer  of  the  epi- 
dermal cells  at  its  tip;  also  the  vascular  supply  of  the  nectary  and  of 
the  subtending  leaf.  The  leaf  is  just  being  cut  off  to  leave  a  relatively 
small  leaf-scar.    X  45. 

Fig.  57.  Surface  view  of  upper  half  of  a  mature  spicule  or  glochidium,  showing  the 
barbs  closely  resembling  those  of  spines.     X  72. 

Fig.  58.  Longitudinal  section  of  basal  fourth  of  a  nearly  mature  bristle,  showing 
internal  structure,  barbs,  and  zone  of  rupture  at  which  bristle  breaks 
off.     X  125. 

Fig.  59.  Transverse  section  near  middle  of  an  immature  spicule,  showing  the  large, 
thickened  surface  cells  which  are  to  give  rise  to  the  barbs.    X  225. 

Fig.  60.  Longitudinal  section  of  upper  half  of  opening  flower,  showing  the  most  fre- 
quent location  of  the  abscission  layer,  which  cuts  off  the  whole  perianth 
from  ovary,  but  leaves  style  to  be  cut  off  independently.     X  3. 

Fig.  61.  Section  similar  to  that  in  figure  60,  showing  another  type  of  abscission  layer, 
which  cuts  off  the  style  as  well  as  the  perianth.  Separation  has  already 
occurred  in  the  left  half,  while  the  abscission  cells  on  the  right  are  in 
the  stage  shown  in  figure  67.     X  3. 

Fig.  62.  Part  of  longitudinal  section  of  wall  of  an  ovary  from  which  the  perianth  has 
fallen,  showing  surface  left  after  the  break  through  the  youngest  cells 
of  the  abscission  layer.  The  arrow  is  parallel  to  the  basal  part  of  the 
style.     (Cf.  figure  67.)     X  48. 

Fig.  63.  Part  of  radial  section  of  perianth-scar  of  very  young  fruit,  showing  phellogen 
layer  and  cork.    Abscission  surface  at  right.     X  117. 

Fig.  64.  Part  of  radial  section  of  perianth  scar  of  a  two-year  old  fruit,  showing  the 
parenchyma  of  fruit  at  left,  phellogen  in  the  middle,  and  cork  at  right. 
X  110. 

Fig.  65.  Small  portion  of  section  similar  to  that  in  figure  64,  showing  structure  of  two 
of  the  thickened  cells  in  the  cork.     X  225. 

Fig.  66.  Outline  of  vascular  system  of  inner  petal.    X  5. 

Plate  7. 

Fig.  67.  Part  of  the  radial  section  of  ovary  of  newly  opened  flower  illustrated  in 
figure  59,  showing  various  stages  in  the  division  of  cells  that  are  forming 
the  abscission  layer.    The  arrow  is  parallel  to  the  style.    X  110. 

Fig.  68.  Part  of  radial  section  through  ovary  of  an  opening  flower,  showing  the  rela- 
tion of  the  abscission  layer  to  the  vascular  bundles  and  to  the  mucilage 
cells.    X  19. 

Fig.  69.  Part  of  transverse  section  of  two-year  vegetative  joint,  taken  through  a 
tubercle  just  below  its  areole,  showing  the  structure  of  the  epidermal 
and  of  the  palisade-like  chlorophyll-containing  tissues  of  the  stem. 
X45. 


THE   FRUIT   OF   OPUNTTA   FULGIDA.  61 

Fig.  70.  Transverse  section  of  a  two-year  vegetative  joint,  at  about  2  centimeters 
above  base,  showing  vascular  bundles  and  the  radiate  arrangement  of 
the  palisade  tissues  in  each  tubercle.     (Cf.  figures  39  and  43.)     X  1.5. 

Fig.  71.  Part  of  transverse  section  of  tubercle  of  an  unopened  flower,  showing  epi- 
dermis, stomata  and  palisade  with  its  intercellular  air  spaces.     X  205. 

Fig.  72.  Small  portion  of  transverse  section  of  two  or  three  year  old  fruit,  showing 
details  of  stomata  of  the  epidermis  and  its  thick-walled  underlying 
layers.     X  365. 

Fig.  73.  Small  portion  of  transverse  section  of  tubercle  of  similar  fruit,  showing 
resemblance  of  the  palisade,  etc.,  to  that  of  the  vegetative  joint.  (Cf. 
figure  69.)     X  110. 

Fig.  74.  Part  of  tangential  section  of  tubercle  of  a  two-year  vegetative  joint,  showing 
palisade  cells  and  mucilage  cells  in  cross-section.    X  85. 

Fig.  75.  Section  of  one  of  the  crystal-holding  cells  so  abundant  in  the  subepidermal 
and  parenchymatous  tissues.     X  365. 

Plate  8.     Photographs  76  to  79,  0.  fulgida;  80,  Caixistemon;  81,  O.  ^-ersicolob. 

Fig.  76.  Lateral  view  of  a  five  months'  seedling,  showing  hypocotyl,  cotyledons,  and 

first  joint  of  stem.    Seed  planted  April  27,  1917;  photograph  of  living 

seedling  made  October  12  following.     X  0.9. 
Fig.  77.  Part  of  a  cluster  of  fruits  collected  at  Tucson  in  April  1915,  showing  one 

chain  of  14  links  in  a  single  linear  series.     X  0.3. 
Fig.  78.  Plantlet  10  months  old,  developed  in  a  greenhouse  in  Baltimore  from  a  fallen 

fruit,  showing  position  of  adventitious  roots  and  structure  of  the  new 

shoots.     X  0.66. 
Fig.  79.  One  of  the  two  examples  found  in  which  the  persistent  fruits  of  0.  fulgida, 

while  still  attached,  proliferate  to  vegetative  branches.    Collected  and 

photographed  at  Tucson  in  April  1915.     X  0.45. 
Fig.  80.  Branch  of  Callistemon  speciosum,  with  three  generations  of  flowers  and 

fruits,  showing  the  persistence  and  growth  of  the  firm  green  capsules. 

Flowers  at  the  tip  about  to  open.    Collected  on  the  campus  of  the  Uni- 
versity of  California  in  March  1916.     X  0.23. 
Fig.  81.  Gall  fruit  of  0.  versicolor.    Collected  and  photographed  at  Tucson  in  April 

1915,  showing  the  curling  of  the  abortive  petals  and  the  fly  Asphondylia, 

which  has  just  escaped  from  one  of  the  pupa  cases  projecting  from  the 

side  of  the  gall  below  the  fly.     X  0.3. 

Plate  9.    Photographs  of  0.  versicolor. 

Fig.  82.  A  branch  of  0.  versicolor  bearing  several  clusters  of  fruits  and  galls  of  1914, 
and  groups  of  flower  buds  of  1915.  Some  of  fruit-like  structures  are 
clearly  galls,  but  others  are  nearly,  if  not  quite,  normal.  Collected  and 
photographed  at  Tucson,  May  1915.     X  0.45. 

Fig.  83.  A  piece  of  a  vegetative  joint  of  0.  versicolor  collected  at  Tucson  in  April 
1915,  showing  3  types  of  persistent  fruits  or  galls.     X  0.6. 

Fig.  84.  Four  types  of  gall  fruits  of  0.  versicolor,  showing,  at  left,  a  slightly  devel- 
oped perianth;  and,  at  right,  a  very  liighly  developed  perianth.  Col- 
lected and  photographed  at  Tucson  in  April  1915.     X  0.45. 

Fig.  85.  Branch  of  0.  versicolor  with  two  galls  of  1913  or  1914.  One  of  these  bearing 
two  vegetative  branches  developed  in  1914,  and  each  of  these  a  cluster  of 
flower  buds  for  1915.  Collected  and  photographed  at  Tucson  in  May 
1915.     X  0.45. 

Fig.  86.  Branch  of  O.  versicolor,  showing  persistent  fruit  (normal?)  bearing  a  vege- 
tative branch  which  is  directed  backward  toward  base  of  the  parent 
branch.    Tucson,  April  1915.     X  0.45. 


62  THE   FRUIT   OF   OPUNTIA   FULGIDA. 

Plate  10.     Photographs  of  O.  discata,  0.  cylixdrica,  O.  leptocaclis,  and 
o.  catacantha. 

Fig.  87.  Portion  of  joint  of  O.  discata  bearing  a  gall  fruit  with  projecting  pupa  cases 

from  which  the  cactus  flies  have  just  escaped.     X  0.45. 
Fig.  88.  Tip  of  a  joint-fruit  of  0.  cylindrica  of  1913,  bearing  a  persistent  fruit  of  1914, 

two  flowers  and  a  vegetative  joint  of  1915.     Note  constriction  only, 

instead  of  the  usual  distinct  articulation  at  base  of  the  parent  fruits. 

X  0.45. 
Fig.  89,  a,  b,  c.  Three  branches  of  0.  leptocaulis  bearing  fertile  fruits  of  1914,  sterile 

fruits,  various  forms  of  more  or  less  fruit-like  branches,  and  also  (c) 

flower  buds  for  1915.     X  0.45. 
Fig.  90.  A  single  joint  of  O.  catacantha  bearing  four  chains  of  fruits  (all  sterile), 

showing  one  chain  of  six  links  on  right  and  a  single  open  flower  on  left. 

Collected  at  St.  Thomas,  Virgin  Islands,  May  1915.     X  0.45. 
Fig.  91.  Series  of  fruits  of  0.  catacantha  (from  same  collection  as  those  in  figure  90) 

in  surface  view,  also  in  longitudinal  and  transverse  section,  showing 

perianth-scar  and  small  sterile  ovarian  cavity.     X  0.45. 

Plate  11.     Photographs  of  Peireskia,  Opuntia  rxjfida,  0.  arbuscula,  and 
O.  leptocaulis. 

Fig.  92.  Two  generations  of  flowers  of  Peireskia  guamacho,  showing  proliferation 
of  primary  flower  from  axillary  buds  at  the  level  of  the  ovarian  cavity 
(indicated  by  an  x).  The  primary  flower  and  the  secondary  one  at 
right  are  shown  in  median  longitudinal  section.     X  0.6. 

Fig.  93.  Flower  and  fruit  of  Peireskia  guamacho,  showing  umbilicate  fleshy  fruit 
with  areole  on  right  side  above.     X  0.9. 

Fig.  94.  A  joint  of  0.  rufida  bearing  three  persistent  fruits,  from  one  of  which  a  new 
vegetative  joint  has  arisen.     X  0.45. 

Fig.  95.  Two  lower  branches  of  0.  arbuscula  bearing  persistent  fruits,  and  these,  as 
well  as  the  normal  vegetative  joints,  bearing  slender  condensed  shoots 
with  closely  packed  areoles.     X  0.45. 

Fig.  96.  Branch  of  0.  leptocaulis  showing  fruits  and  fruit-like  branches,  like  those 
described  for  figure  89.     X  0.45. 

Plate  12.    Drawings  of  O.  fulgida. 

Fig.  97.  Tangential  section  of  half-grown  seed,  perpendicular  to  the  flattened  sides  of 
seed  and  passing  through  micropyle,  showing  pocket  of  funiculus  which 
incloses  the  seed  and  the  two  integuments.    X  95. 

Fig.  98.  Section  of  ripe  seed  in  plant  of  greatest  diameter,  showing  embryo,  endo- 
sperm, integuments,  and  thick  wall  of  funicular  pouch  with  its  included 
vascular  bundle.     X  17. 

Fig.  99.  Longitudinal  section  of  a  fruit  which  has  given  rise  to  a  new  plantlet,  show- 
ing the  portion  of  the  fruit  cut  off  by  a  layer  of  cork  from  participation 
in  the  new  plant.    X  1.33. 

Fig.  100.  Longitudinal  section  of  a  fallen  fruit  through  the  point  of  origin  of  an 
adventitious  root  from  the  border  of  an  areole.    X  14. 


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